What Character in a fighting game you found to be very hard to master?

As some might know, Im big on fighting games. And pretty good at them I might say. But sometimes you come across a Character that seem kinda hard to master. When I say master, I dont meen a character to cheese your way through with. A fighter that require skillz to get you through.

The characters I found hard to master
1. AOI, Vanessa, Akira, and Shun from the VF series.