I'm not gonna avoid spoilers because I don't really think this is worth reading.
So, to start with, rather than Haruhi Suzumiya, this seems all about the narrator, whose full name we don't learn, just the nickname Kyon, which apparently means something similar to 'Bambi' (it refers to a species of barking deer, and the n makes it a feminine name). His unique characteristics are non-existent, as being an awkward teenager and wanting to believe in paranormal things but not having enough evidence are not, imo, unique or even exclusive characteristics. Also he's a horny teenager. His development by the end of the novel is, as far as I can tell, minute to non-existent in terms of agency, honourable action, or innate, unsubstantiated belief in paranormal people/creature.
Haruhi Suzumiya. I can't really stand her as a character. At her core she's a girl looking for magic in the world, which I can't take issue with, but every single thing about her seems designed to grate. She's inconsiderate, rude, unobservant, and excuses her actions because she feels she has a higher purpose. She gropes and attacks other people either for comedic effect, for the sexual gratification of the readers and author, or because at some level she understands and agrees with the central idea of the book, which is that the entire world revolves around her. Her romance (is this a romance? It seems like an ignorant crush or high school fling.) with Kyon seems to be based on the fact that he is slightly less boring than everyone else rather than because he is attractive or more interesting or a good match for her.
The unique way all of the paranormal entities are presented bothers me a little. Haruhi seems like a girl completely set in her beliefs, with definite aspects of phenomena that she's looking out for. I acknowledge the possibility that she could have come up with original takes on aliens, 'espers' and time travel but, this seems more likely a consequence of the author wanting something original to engage the readers than a reflection of subconscious assumption and conflation of things she's read about and hopes to see. I admit I'm not really up on Japanese media and myth which does, at a glance, seem keyed to innovation and originality, but it clashes with Haruhi's character as I understand it. Her disbelief, which leads to the major conflict, stems from a strong set of beliefs being unsubstantiated rather than a fluid hope which would by it's very nature, be perennial.
This seems like a fairly accurate portrayal of the mind of a high school boy in that Kyon can't seem to make up his mind about which girl he's attracted to and/or wants to be in a relationship with. Less realistic that he's surrounded by incredibly attractive girls all with some sort of emotional investment, and the girl whom the world revolves around is secretly in love with him, along with the loli that is part of his blossoming harem. The only other guy who isn't a minor character is a pawn of Haruhi and seemingly not a threat romantically or sexually for anyone's attentions but rather an impartial observer.
The ending - not really a conclusion or resolution - of the novel is a moment where Kyon is forced to kiss a hot girl who annoys and attempts to control him or the destruction of his world as he knows it and it being recreated more to Haruhi Suzumiya's liking. Which would probably be fairly similar to what it was already except more obviously paranormal and he would have gone through all of this disturbing stuff and have been transferred from one world to the other as Haruhi's crush. It's not a culmination of his feelings for her, she would seem to have shallow half-formed feelings for him, and honestly I'm not sure it would have been a bad thing for the world to have been made anew in the way of a muscle tearing to build up or a creature evolving to become greater.
I think this book was a mistake. The fact that it's a romance, that it revolves around the male unnamed narrator, the methods of Haruhi in searching for paranormal phenomena, the proliferation of actual paranormal phenomena. It would have been so much better if this was a story about Haruhi alone, unlinked romantically, believing without evidence in a seemingly barren world, gradually learning to treat people better, accepting the idea that maybe she should improve her methods of observation. If the whole world did not mystically revolve around her because she was in fact, magic or special in some huge cosmic lynchpin way, that would have improved her character immeasurably. The pace leaves no room for her to develop a character. She goes from baselessly believing she is better than everyone to, by the extradimensional climax, feeling out the edges of the base of her being an actual god/time anomaly/creator of life and exemplar of evolution.
The prose isn't terrible. It's not perfect and could use some work. The main issue, though, is with the pacing of the events as it relates to negative character development, and a lack of depth in characters as a whole. I feel that someone like Seanan Maguire, or Patrick Ness would have made this into a solid, deeply relatable work that I would read over and over again.
The last Japanese thing I read was Lone Wolf and Cub, and the book I read directly before this one was The Book of Three by Lloyd Alexander and the contrasts between, respectively, character depth, and the complexity and realism of the relationship between Taran and Eilonwy aren't favourable at all to this book, so I may be judging too harshly, but shouldn't you judge something as it stands in literature as a whole rather than as it stands in it's genre?
Plus this is like the first ever light novel I've read so I can't really judge it by it's genre.