Castle Swimmer: Volume 4 [A Graphic Novel]

Castle Swimmer #4

by Wendy Martin

Kappa and Siren descend deeper into the darkness of the ocean and uncover the harrowing truths of their own destinies in this thrilling fourth installment of the hit Webtoon series Castle Swimmer.

Kappa and Siren continue their thrilling journeys beyond the kingdom walls. Accompanied by his new companions Pim and Pagoon, Siren ventures into the shadowy ocean depths in search of a cure for his curse. Facing terrifying creatures that prey on their deepest fears and desires, the trio uncover more about Siren’s prophecy and the darkness entwined with his fate.

Meanwhile, Kappa continues to battle the great demigod crab with the last of the Octalia people, learning to harness his magical powers and grow his strength. Despite being on separate missions, Kappa and Siren remain connected through their unwavering dedication to each other, even if it is just an inflicted hallucination.

Join Kappa and Siren as they bravely push forward into the unknown and face more mythical obstacles in this captivating continuation of the Castle Swimmer saga. This beautiful graphic novel collects episodes 66-86 of Webtoon’s Castle Swimmer.

Genres: Graphic Novels, Fantasy, LGBT, Comics, Romance, Queer, Fiction

288 pages, Hardcover

Published September 8, 2026

Reviews

[✨sofia✨]

I love this webtoon sm

[Alyssa]

Thank you to NetGalley and the publisher for giving me an ARC of this graphic novel in exchange for an honest review!

3.5 stars.

This one definitely seemed like it was a bridge volume. Something we need to get past, but not as exciting. It was kind of info-dumpy, with a little bit of action.

I flew through this even still, and I still very much love the art style, so I will continue to read these as they come out! I am very excited to see where the story goes. This one was just a little more boring to me,

We didn’t see as much of Kappa, which makes me sad we didn’t see him, but I still don’t love the “Evil Witches” yet, so I am okay with not seeing their plotline as much.

I love Pim and Pagoon, their relationship with each other and with Siren is so sweet!

Arp steals the show as always! He is so cute! I was melting. I like how we saw a little bit of his back story.

I also loved the ending, where we saw all the moments between Kappa and Siren! That is what made me raise my rating half a star. They are so cute with each other. I love how blushy they get! I need so many more romantic moments between them!

I am excited because I am hoping we will finally get Kappa and Siren reuniting next volume! At least Siren will work his way to him. I need them to be together again. That is what I am mainly here for. The curse breaking stuff is fun, but not why I am so invested in this story.

[Rhys]

Thank you to the publisher for a copy via NetGalley.

Castle Swimmer is such a cute achillean graphic novel series! I’ve enjoyed all volumes so far.

My only thing I wish we had more of was Kappa. This volume was heavily focused on Siren (which was very fun), but we didn’t see that much of Kappa. I wanted more!

[ivanareadsalot]

I would like to thank NetGalley and Ten Speed Graphic for the opportunity to read and review this ARC.

✨Castle Swimmer: Volume 4 collects Episodes 66-86.✨

🐟🐠🦈🐚I flew through this latest volume in Siren and Kappa's adventures. The gorgeous art continues to charm me, and the dynamism in the character art and the storytelling flow kept the vibe energetic and punchy. And the cast of engaging and affable characters just keeps growing more lovable the more I get to know them. Siren continues to be an emotionally gripping sweetling, who's still processing what had transpired between him and his mum in the last volume, and missing Kappa too ofc.

🪸🪼🐋🦑Lots of action kicked things off, as both Siren and Kappa resolutely honed their skills training with their respective masters while battling fearsome creatures. They both ended up learning more about their party's pasts and that had all of us up in our feelings. The ocean is turning out to be a lot more curse-filled and dangerous, especially to an adorable shark prince and his mythical beacon crush! Kappa wasn't in this volume too too much, but where and when he did show up, he shone brightly as an impactful, endearing and adorable beacon of love potential as he is wont to do!

🦈🦞🐚The monsters were creepy scary in this one, most especially the cursed ones Siren and his master, Pagoon, had to fight to get warrior mama Pim back. Siren ditched his knife and leaned into his naturally vicious and effective biting mode, and then went through some deep, dark-night-of-the-soul things when curse illusions threatened to break him. But he stood firm on what he believed to be true, and eventually overcame his negative thought spiral and bad memories.

🐟🦪🐠And after learning of yet another abysmally cursed people and being sympathetic to their plight, Siren added them to his list of everyone who needs saving. He would continue to aim for that to happen without anyone (him or Kappa) having to sacrifice themselves so others could live.

🦈🧜🏽Siren just wants to travel and live his life and fall in love with his sparkly bae AND I REALLY REALLY WANT THAT FOR THEM PLEASE OMG!!!

🐟Special shout-out to darling companion Arp who loves both Siren and Kappa omg I was teary eyed at how precious the ending was.

🪸🪼🦀🐙This story just keeps getting better and better and I can't wait for Siren and Kappa to reunite! Siren's memories of their time together were so smushy and they very much deserve to find happiness and true love with each other ASAP! Castle Swimmer: Volume 4 was just as stunning as the previous three volumes, and I CANNOT WAIT to see where this awesome story goes next!

[librariSAM]

Thank you NetGalley and the publisher for the opportunity to read an e-ARC in exchange for a review.

When I saw this was on NetGalley, I requested it immediately! This is one of my favorite graphic novel series I have ever read and this newest volume did not disappoint at all!

Siren continues to be so likable that I want to protect him at all costs. We get less of Kappa this volume but what we got of him I really liked. He is so adorable, I want to put him in my pocket.

I love how Siren opens up to Pim and Kappa starts to get to know the witch crew better. I love the found family trope and I'm really glad it is shining in this series. I am scared to death something is going to happen to Pim now that she knows everything but I really hope not.

The storytelling was amazing as always. I love how we get to learn about the different sea creatures and their stories, it really adds to the worldbuilding in this series and makes it even more interesting to ponder how this will end not only for Siren and Kappa, but the rest of the cursed creatures.

I also have to mention the art. 10/10 as always, but I already knew that going in. The cohesive color palette makes this art fully unique to this story and makes you feel like you are really under the sea with our characters.

I also loved the memories at the end of this volume. They were SO ADORABLE and made me smile so hard. It was a good idea to give us some Siren and Kappa moments since there weren't any in this volume.

This is the 4th volume and I've recommended every volume prior, so if you are somehow reading this review and haven't read the first 3 volumes, I recommend you get started ASAP. I would recommend this to ANY graphic novel lover, but especially someone who enjoys queer stories and absolutely lovable characters!

[Snarky Hatter]

Castle Swimmer Volume 4 focuses on character growth, lore, and raising the stakes for our heroes Siren and Kappa. Siren in particular gets to shine in this volume, while Kappa’s story takes more of a backseat.

Siren, who spent the last volume being trained by Pagoon, has become a better fighter, but still struggles with wielding a knife. This becomes a metaphor about him embracing himself, or to put it more literally, it’s about him leaning into his shark instincts and biting his opponents instead of relying on his knife. Obviously, the metaphor about being true to oneself is far more poetic than what literally happens, but reading Siren chomping on giant sea creatures is nonetheless fun.

Without going into in-depth spoilers, through Siren the reader learns his curse isn’t the only thing he must break in order to save himself from his tragic fate. The scenes leading up to this reveal are very poignant and evocative, while also introducing a new class of merfolk, who are beautifully designed.

Meanwhile, Kappa learns more about the troupe of “evil” sea witches he’s joined, and why they are in pursuit of a mini-god.

For those who love a fast-moving plot, this volume might move too slowly for you. However, anyone who loves character depth and exploration will gobble up this book.

Thanks once again to NetGalley for an Advanced Reader Copy of this book. It’s been wonderful to read Wendy Martin’s story. Not just because of the plot and characters, but also because of the simple beauty of her art.

[DatHeetJoella]

The plot really thickens in this one! The lore about the mini-gods, the curses and the prophecies affecting all the different communities is explored and explained in this volume and I enjoyed that a lot. The characters continue to be great; Kappa and Siren are both their charming selves, the three witches are a perfect balance of funny and tragic, and Siren's bond with Pagoon and Pim is too cute for words.

The scene where Siren was caught in the trench and all the illusions were trying to trick him, Siren looked at Arp and hugged him so tightly, that really tugged at my heartstrings. That panel was my favourite of the whole volume.

The series has properly sucked me in and I'm gonna continue reading ahead on Webtoon now, because I can't wait for Kappa and Siren to reunite (I also saw in the preview that Queen Nee and Queen Krilli are going to make a comeback and I can't wait for that either!)

[Emily]

Thank you to Netgalley and Ten Speed Graphic for providing a copy of this comic!

I'm still really enjoying this series! I am finding myself more engaged with Siren's story than Kappa's at the moment. Not sure if this is actually because his story is more interesting, or if it is just because his story was focused on more in this volume. Either way, it was much more engaging. There were also a lot of good emotional beats in this volume. I actually found myself tearing up during a couple scenes. Humor still isn't really working for me, but there was less in it in this volume. Overall, I think this was a great installment. We are actually starting to get more progress in the plot, which is nice. We are also getting some good character growth, mostly from Siren. Can't wait to read the next volume!

[Juan Esteban Duque]

El tomo más profundo emocionalmente de Castle Swimmers hasta ahora. El conflicto interno de Siren entre el deber de cumplir la profecía y lo que su corazón verdaderamente anhela se ve mucho más explorado. Recuerdo una escena donde no pude evitar contener las lágrimas; en esta, Siren habla de sus anhelos, y su mayor deseo es algo tan humano (a pesar de que sea un tritón/tiburón) y eso es vivir, no por su pueblo, no por sus seres queridos, sino vivir para sí mismo, para explorar, para descubrir el mundo, simplemente por el hecho de querer que su corazón siga latiendo. Es tan hermoso explorar esa dicotomía entre querer salvar a los demás pero también salvarse a sí mismo en el proceso. Es un tomo hermoso, un viaje emocional con personajes entrañables; es una lectura ligera a pesar del peso emocional de la historia

[JT Milam]

Wow! first of all, i would die for Arp.

volume 3 had a lot of world building which was cute but now i see it was totally necessary for this insane action packed volume! the weight of everything on siren’s shoulders really came down on him in this volume and im glad he was able to work through some of that turmoil.

we really focused mostly on siren and as a kappa lover, i was disappointed but totally okay with it because our boys will be reunited soon!!

i BLAZED through this in under 30 minutes because it was genuinely non stop action. new monsters, new lore, new locations! castle swimmer forever!!

thanks to netgalley and the publisher for an advanced readers copy (ARC) in exchange for my honest opinions.

[Reader]

Thank you Netgalley for providing me with the ARC of this graphic novel.

We are SO back with Siren and Kappa! Well, not so much of Kappa as this volume was heavily focused on Siren but OH MY GOD IT WAS SO ADORABLE.

This webtoon (before it became a graphic novel) was something I started for fun with no expectation of any plot. I had expected it to be some cute romcom but make it between ocean creature. But OH the plot is plotting in this volume.

We get LOTS of lovable character, Pagoon, Pim, the witches, and of course, Arp along with Kappa and siren.

AND you have small bonus chapters at the end of the volume with scenes from the time where Kappa and Siren were in the castle. Spoilers: They hold hands! Scandalous!

[Phoebe]

Thank you to the publisher and NetGalley for the ARC.

Still cute, cozy, and comforting, but I was glad to see this volume explores some darker topics relating to the characters' traumas, giving the storytelling a bit more bite and emotional depth. I will also say the designs of the trench creatures were actually pretty creepy to me, which I wasn't expecting considering the more cuddly designs in the webtoon overall, so that was well-done. Looking forward to Siren and Kappa's eventual reunion!

[fireheart]

Read on Webtoon!

I think this one is my least favourite so far, but still great. I loved seeing more with Siren, Pim and Pagoon. It was very sweet seeing them bond. I do also enjoy learning more about the evil witches, but definitely not as much as Pim and Pagoon. I miss Siren and Kappa together so badly, really hoping they reunite soon. As the other volumes aren't out yet, I will be reading the rest on Webtoon!

[Nina]

Thank you to Clarkson Potter/Ten Speed Press and NetGalley for a digital ARC of this book.

These covers are so misleading 😭 I keep hoping Siren and Kappa meet up again because they're together on the cover.

Still. The story is so cute. I am loving the growth our major and minor characters are going through. I do enjoy that Siren and Kappa are learning more about themselves and thr world before they meed up again.

[J]

I received this DRC from NetGalley.

This is an interesting continuation of the story. Kappa and Siren are still having separate adventures with their new companions, and we get to learn more about their new friends' backstories. Siren's relationship with Pim and Pagoon is really sweet, and I liked that we get to know them better. I think anyone who is already invested in the story will enjoy this volume.

[reis_readingg]

4.75/5
I genuinely LOVE this series so much. I’ve been reading the webtoon since pretty early on, and i love seeing the novelizations. This one is adapted so well, and genuinely holds some moments that are so sweet. Definitely missing siren and kappa, but overall a great book. Also, loving the little memory flashbacks!!

[Tye Rose]

Volume 4 is as beautiful and sweet as expected from the other GNs. I enjoyed seeing more of Siren, but I very much missed Kappa in this one.. He had very little page time. I loved the little memory comics at the end! Arp is still my favorite character.

Thanks to Netgalley & the publisher for this arc!

[Sophy]

This story is always such a cute read. I love getting to watch the characters grow as they each go on their own journeys in search of answers.

Following their adventures and learning more about the curses has been a lot of fun, and I can’t wait to see what happens next. The art is always adorable and complements the story perfectly.

[Juliana]

I loved getting more of the backstory on Siren and Kappa’s traveling companions in this volume. It was one of the more emotional volumes so far. There seems to be a common problem among all of the mer-people. I wonder how it will be addressed in each situation. I can’t wait to read more. I highly recommend this series!

[booksonthefloor]

Look, we are all suffering from a very large Nick-and-Charlie shaped hole. But Castle Swimmer is SUCH a good choice if you need to fill the void. Loveable characters, good storyline, and a lot of heart!

[Marvelous]

This section of the web comic is so wholesome. Thered a bit of exposition that comes of middle-grade-y (...probs bc it is middle grade) but it also revamps some old tropes like the "stuck in a fog of corrupted memories that messes with my sense of what's real and what's not."

I love gay fish.

[Carrie Griffin]

My favorite volume from the series so far, especially the memories. It was great getting a second adventure from Kappa and Siren this year and seeing more of their journeys with their companions. I also loved getting to know more about the world and the other creatures.

This volume deals a lot with fears and desires especially with the creatures Siren, Pim, and Pagoon come across. Those scenes were so interesting. I enjoyed getting even more world-building.

I'm highly looking forward to seeing what will happen next in the future volumes from this series.

*Thank you to Ten Speed Graphic, Wendy Martin, and NetGalley for my ARC of this book. All opinions are my own.*

[Brenda Duong]

Thank you to NetGalley & Ten Speed Press for the ARC!!

I remember starting this series on Webtoon during the pandemic so I was very excited to get dunked (😏) back into my middle school years. The artwork is every bit as lush and calming as I remembered. The color grading did an excellent job of cueing me into the mood of each scene and how I should expect to be feeling. Of course, I would recommend reading the previous books, but I followed along easily enough for someone who hasn't seen Siren and Kappa for quite some time.

Volume 4 follows Kappa and Siren's individual journeys, though they were reminded of each other each step of the way. This did focus more in Siren, and we got to see a more vulnerable side of him, along with what his fears and anxieties have amounted to. Side notes: I had forgotten how funny Pagoon is, who reminds me of a younger sibling & getting to see emo rocker bangs Kappa was not on my bingo card!

𑣲𝄞SONG OF THE BOOK:
Holding On (by Dabin, Lowell)
0:47 ━━●──────── 03:37
⇆ㅤ ㅤ◁ㅤ ❚❚ ㅤ▷ ㅤㅤ↻

[Cara Newman]

I am reading this on WebToons and the storyline just keeps getting better and better. I find myself gasping in surprise or laughing randomly. I seriously love this series

[Stella Blackwell]

Read this on webtoon and it was soooo good!!!

[Kennedy]

Cuteee I love this series

[gray (my.rainbow.bookshelf)]

This volume got a lot darker and heavier than I was anticipating but expanded the lore and storyline in such a great way that made me even more excited to keep reading

[Katelin]

Review to come week of publication.

[Maple wolf 2012]

I read this on WEBTOON.

[Audet Maxime]

This story keeps delivering great actions, cute moments, and deep worldbuilding. Always a pleasure to read and see where the author takes us.