Alright, while fooling around in practice mode with V. Akiha, I found that 4, 236C activated her arcdrive. Seeing as I've played on keyboard for a while now, I wondered why I hadn't discovered this earlier.
However, when trying it with Nero, 4, 236C always gave his EX crows, and only 41236C activated the arcdrive. I'm sure my input was a close to identical as I could possibly make it, yet I got an arcdrive 95% of the time with V. Akiha using 4, 236C and never (unless I "messed up" and did 41236C instead) with Nero.
So, the question: Does different characters have different rules about input (varying priority on which specials come out?), or did I magically get awesome keyboard crushing skills whenever I picked V. Akiha?
EDIT: For added clarity, I added two replays. Turn on input display and watch them. The input is near identical, yet obviously different moves come out.