Voldo might be good actually, I used him a lot in 2, and I haven't a CLUE how to fight him in this game. And Onyxx, you could, but unless I can put him in a dress and give him twin drills, he's just not the Twilight for me.
How about sticking to who tickles your fancies more? I stick to Yoshi and Leixia, but imho I don't have tertiary. This is because I'm deciding between my scrub subs Natsu, Pyrrha, Mitsu, Asty, Hilde, and Damp. Now you know the meaning of pain.