Originally Posted by
Synaptic
Anybody that can bully him; Karthus can't really handle constant pokes and harassment in lane, and much prefers to just have a farm-off. Champions that can constantly get to him and poke on him will give him issues, because Q just isn't the most reliable way to even out trades.
Soraka, Kassadin, Leblanc and Kennen, among others, give him fits.
Contrary to most other mages, however, Karthus fully benefits from laning against Morgana(or any other non-bully). She simply cannot prevent him from farming his way to hyper-carry status.