Sabaki Ryuuken (Azcal)-He can counter anything you throw at him including desperation moves, he has a move similar to the Raging Demon and depending on how much HP you have it's instant death.
Saiki (Werewood)-I haven't played KOF XIII but i doubt the source game is super cheap like this one.
Ikaruga's characters-Almost all of them have super cheap AI that revolves around comboing the shit out of you nonstop like it's Killer Instinct. X___X
Muteki's characters-His Guilty Gear conversions are crazy to fight against. His I-No and Ky Kiske suffer from piss easy A.I though.
Shotoborg (Kamekaze)-Has high damaging combos and shit.
Wolverine (Sander11113)-Very aggressive when it comes to pulling off all sorts of rushdown combos.
Sephiroth (Gulthor)-His attacks are highly damaging and that teleport move of his is FUCKING ANNOYING....Gulthor is obviously one of Sephiroth's fanboys who think he's an almighty god which is why he made him so fucking overpowered.