We meet four Slithers throughout the story, and not a single one of them is a good villain.
Solon-Token Gharnef Rip-Off. Shows up, does a thing, and dies immediately after. Thrilling
Kronya-Token Fanservice Chick As much as I love her design, I must admit that this character is wasted potential. She shows up, KILLS YOUR DAD, and than dies immediately after. If you're going to have a villain do that, they should be a serious enemy that the protagonist and player grow to hate, not a glorified mini-boss who is killed off immediately.
Cornelia-Token Fanservice Chick 2.0 since Kronya died prematurely. She's just introduced through exposition halfway through the game. In one route she's partially responsible for the terrible state the protagonist is in, when he reclaims his kingdom from her it's a major turning point, in one route she's just another glorified grunt that you kill, and in the other two she's only mentioned. Ok in one route I guess, nothing in the others.
Thales-Mr Evil Warlock Behind it all. Y'know, for the game'a true villain, he's hardly present, and hardly even memorable. He just makes a few scattered appearances until he's resolved off-screen in one route, sorta and vaguely resolved by you killing his human form in one route, and only actually fought in two routes. We learn absolutely nothing about him other than he wants revenge, which is a common goal among everyone in his group. We also have no reason to care about him, since he doesn't really do anything. Honestly he should've been the one to kill Jeralt since the player would have a reason to care about him.
Not a single one of these four is an interesting character in the slightest. Like, they look cool I guess(aside from Cornelia)? The existence of the slither gang brings up so many questions that go unanswered, and they're responsible for the existence of most of the game's plot holes. Do they really add anything? As they are right now I would've preferred the game not have them.