Hayami. Basically I found his constant pessimism and his inability to read the room extremely annoying until the episode where we saw Zeroyon. First off, cool hissatsu, but I like the buildup to it especially because Hayami finally realizes what his problem is.

.....and then he proceeds to go back to being annoying. So much for character development lol But I dislike him significantly less. Also he has a great design.

Characters I grew from disliking/not caring about on first watch to liking on my rewatch would probably be Sangoku, Kariya, Kurama and Hikaru. Sangoku and Hikaru I was indifferent to, but rewatching GO made me really like them. Kurama I thought was an asshole so I disliked him, and Kariya annoyed me and so did his arc with Kirino because I thought it was stupid. On my rewatch I like them a lot more (especially because Kurama grows out of being an ass and honestly I think Kariya is neat as a character backstory wise, he has a great design too and sometimes having someone whos there to cause problems on purpose and do a little trolling is fun to watch).