The S_SKIN is a special text lump used to define characters in both SRB2 and SRB2 Kart.
Here's an example of how it looks like from Vannila SRB2 :
name = Idreia
ability = CA_DOUBLEJUMP
ability2 = CA2_NONE
normalspeed = 36
thrustfactor = 3
accelstart = 112
acceleration = 42
startcolor = 200
prefcolor = Wave
supercolor = Sky
prefoppositecolor = Peridot
jumpfactor = 1
SRB2 Kart also needs the S_SKIN text but it's different from the vannila srb2. The only thing that doesn't change is the name and the prefcolor iirc. You can check how to do a character by viewing other characters add-ons to see how they work or you can use
Kartmaker. If you want to know more about the S_SKIN, you can click on
this link. The wiki has all the information you'll need.