I think Klaus developed pretty good in season 1 with his powers

Definitely, Klaus. He went from a drug addict to becoming nearly sober to find his partner Dave, and was able to (nearly) bring back Ben to the point he was able to physically punch Klaus.

And of course season 2, again he became completely sober and explored his powers more. Along with Vanya, since she learned to control her powers. Personally, I feel like Klaus was really developed within the two seasons, with Vanya coming in second and Luther third.

I feel like they gave klaus a lot of potential at the end of season 1 but didn't develop it as much as they should have. Vanya has developed the most in such a short amount of time compared to the others. At the end of season 2 they left a lot of potential development for deigo's ability.

Yeah Diego's power is so much more than knives.

Klaus

5 stayed the same, while Luther changed his self status

Yeah, people don't usually change much in two weeks.