Jekyll and Jasper might be drift compatible . . . ? But the both of them would make terrible pilots. With their powers combined, their terribleness would grow exponentially. They would make a Jaeger who is only good at property damage and tripping.
Jasper's human design has changed the least of any character I've ever developed for TGS. I've had a pretty clear image of him ever since he was a minor character in the original draft of the story and he's stayed pretty much the same since I did some preliminary attitude sketches of him last year. It helps that he has a lot of prominent behaviors that affect his posture and the general way in which he moves. In particular, his appearance is based on a description my ex-girlfriend once gave about her cats: hours before their regular mealtimes, they would start following her around, giving her a look that said, "We've never EVER eaten before!"
(P.S. it's my birthday today, yo)