I have the same problem on my laptop. It seems that only we laptop users have this problem?
I cheked the Task Manager and the memory usage doesn't change at all between the character selection screen and the fight, but nevertheless during the fight it jumps from 12 fps of the menu to 30 fps.
However I noticed that when I higlight a character on the selection screen, and the 3D model is loaded, the memory usage goes up by about 100mb, for every character, and it doesn't go down, not even during the fight.
So yes, all character models remain loaded at the same time.
Maybe I should also add that my computer overheats after playing this for 15 minutes.