I've been using Rooki, but I noticed that each character has a different speed/weight stat in the tips of the game. What is the character with the most speed?

It's not actually said anywhere as far as I know, but from my tests it seems like Poppyto is the fastest character. It's not super significant - if Poppyto can do a race in 1:00, then Rooki can do it in like ~1:01.5.

A much more important factor to your speed is making sure you're playing on Expert mode instead of Progressive mode - you drive like 50% faster in Expert mode.

Worth noting that if you're playing the free demo, then the best character you can get is Bigoudi, who happens to be the second-fastest character overall. If Poppyto can do a race in 1:00, then Bigoudi can do it in like ~1:00.5

Thanks for the advice. I've been playing progressive mode the whole time.

Another question, how do you stop from the elephant or frog from bumping into your kart and making you spiral across the track? This happens a lot at the beginning of races.

Unfortunately the only way to avoid that is to play a heavier character. Poppyto is lightweight, so the only surefire way to not spin out is to just not hit the heavier characters.