Actually:
A) Individual characters DON'T have signature builts. It's divided in weight classes.
Heavy characters CPU often use ATVs , or ANY Standard vehicle with Monster Tires.
Like here:
However , this is not exclusive to Rosalina or individual characters as i said.
B) Medium characters prefer standard karts with normal tires or Outward drift bikes.
C) Lighter characters and Medium characters also have the preference for the PIPE FRAME , as seen above.
Finally , according to some research done by an user , MK8 does have a rival system HOWEVER , it only applies to the characters available in the first demo:
The unlockeables such as Rosalina and the Koopalings don't have rivals.