What's the Name of That Book??? discussion

i got this from my primary school library sometime in the late 2000s/early 2010s. it followed a male main character who had a single dad and his friend (maybe sister) who's name was angie. the antagonist was the devil but i remember it had a fairly light/comedic tone? it was more mysterious than horror, i'm pretty sure it had more folklore stuff like contracts and guitars instead of biblical devil lore. i believe it was set in a beach town. there was definitely more than one book in the series. the only scene i remember in detail was the main character and angie growing devil tails and running out into the ocean at the end of one book as a sort of cliffhanger. pretty sure the author was a man as well. i read this book in english but i've seen some similar comments about a dutch book so it could be a translation

Neville the Devil by Michael Lawrence?

Becca's suggestion of Neville the Devil by Michael Lawrence sounds like your book.

Google Books preview:
https://books.google.com/books?id=I9A... There are different cover images.

Quotes from the book:
- 'Maybe he's a devil,' I whispered.
'Doesn't look much of one,' Angie whispered back. 'Could be a retired one.' He looked about as ordinary as anyone could look. Ordinary jacket, ordinary tie, ordinary grey hair sticking out the sides of ...
- So we stood there, half under water, waiting to see if our tails would disappear or if they were a permanent feature. It was a long wait. Such a long wait that I'll stop there if you don't mind. I don't mean stop in the sea, ...