I have a few that I really hate. For the sake of brevity they're all from games available on this site.
Sorcery Jokers
Tōya Rikudō, the final antagonist. I'm pretty sure I already ranted about him in my review of the VN but this guy is the worst. He's a smug pretty boy, effeminate, hates women, has hax powers, is convinced the universe revolves around him and has somehow masterminded everything in the story so he can refashion the world in his image. All the previous villains were his pawns he mind-controlled via his main minion who's hopelessly in love with him (he hates her btw). He's like an unholy fusion of Aizen, Light Yagami and Madara Uchiha. I really wish he didn't exist.
Euphoria
Rika, Rika, Rika. She's a spoiled crybaby who's completely irrelevant to the plot. She does nothing but whine and her ending is completely worthless. She's only there because ClockUp feel they have to include h-scenes involving a loli no matter what. Fuck you Rika, I hope you slit your throat for all eternity.
If My Heart Had Wings
Asa. This one bears explanation. You see, I have a strong dislike for harems. I also dislike threesomes. Asa and Yoru are twins. I love Yoru, she's my favorite character. Asa, on the other hand, is a very generic moeblob. Asa has a route. Yoru
technically has a route... except Asa is also part of the package, despite already having a solo route. Sure she's pretty much a third wheel, but still... It really annoyed me.
Starless
For the sake of honesty, I'll admit I'm not a big fan of Sei Soujo. His art is very much an acquired taste and his writing tends to flip-flop between over-the-top black comedy and the very serious consequences of said comedy. He also has a nasty tendency to force love interests on the player, and in Starless her name is Mitarai Yuuna.
What's the problem with Mitarai? She's insanely boring. She's your stereotypical "class prez" kind of character with huge boobs and glasses and the game
really wants you to love her. All the endings are about what happens to her, your character is literally incapable of being happy without her, it's just terrible... Because it's a goddamn
hard femdom game, not a fucking romance story!
Corruptions of Champions 2
The kitsune. All of the kitsune. Don't feel like explaining why here, no doubt we'll shit on them in the game thread again sooner or later.
Sphilia's Familiar
The MC. He's a walking erection and little else, entirely selfish and without a single redeeming quality. He isn't even good at sex! The NTR route of the game is hilarious by its badness (the ntr dude has a literal mind-controlling dick) but it's at least half-way cathartic to see that fucker put in his place... too bad it involves him changing personality entirely to become your average passive cuck.
That game really sucked hairy balls
Since I'm a robot powered by hate there's probably more to add to this list but this will have to do for now.