[@Crimson Flame] Something that would fit would be a general cap to how fast he can run for long periods of time and a maximum speed for shorter distances before tiring out. Sort of like when you go for a jog you keep a steady pace but if you are doing like a 100m sprint you go as fast as you can but tire by the end of it and have to recover. 

[@Ryik] I like the added details. seems like he is on the border of having multiple personalities, or is that more like being bipolar? Sounds fun to write though, I'm going to have to remember the idea of a character with multiple forms and behaves a different way in each one...