Javier Cross
I dunno how to answer any of those questions, I only get one shot and it ought to count for something good.
I can ask this, though:
How many people in the Guilty Gear universe have their power levels go Over 9000, as the storylines progress?
Meantime, where can we begin with regards to Ky Kiske?
Uhm. Well Slayer is at maximum levels. Always. Although he's mused that several characters like Jam and Venom could surpass him with enough time...however both of them are mere humans so the chances of them becoming overpowered is is low.
Then there's Sol, Dizzy, and Justice. Sol is rarely pushed to go all out though. BUT he is powerful enough to man-handle just about everyone. Dizzy is potentially stronger than Justice but since her innocent personality has her at bay (granted, in the original timeline she commits mass genocide with an army of gears). Justice is mankinds creation, their ultimate doom. But she never really gets more powerful than she already is, unlike most of the GG cast.
May...is a special case. The Japanese race were able to control a very powerful energy (If I recall, it was actually Chi/Ki energy) and were eradicated. May is one of the very few survivors. She is already pretty powerful, despite how young she is. She could potentially have a power level of over 9000 but in the story she is more concerned about proving herself to Johnny.
Other than that...the cast boils down in strength. It's not too ridiculous, really.
Not really sure what you mean about Ky, though. He's a BAMF in game and in story.