Katarina
What makes this champion OP, is not her abilities, or the damage she can do, it's the speed at which she can do it. The only counter to Kat is hard CC, but oh wait, she unloads her full damage before any of your CC can even land. So actually she has no counter at all. Sounds fair. She needs her abilities slowed down. There needs to be a slight pause in between them. The ult should last way longer.
