Our protagonist has become that "half good power half evil power" trope that's much too common and usually done poorly.
However, rather than a "half angel half devil" thing, it's "half fae half devil", which despite being a minor change makes it really refreshing. Plus it's a sign of Seth's character growth and new self restraint rather than the author trying to make a born-special Mary Sue who's just like every other born-special Mary Sue.
also I am all on board for a cute shorthaired sometimes-glasses girl ally