The one-offs in Topaz are:
The starters
Purior
Blitzfaust / Vilephist
Leoracle
Khaphix
Flintusk
Allufang
Snarliger
Kraklaw
The legendary trio
Kuranpu
Typhidna
Unicrown
Kipporok
Jirachi

Is that an inordinate number? Yes. Are those mainly really cool Pokemon that no one wants to see go? Also yes.

Vincent is the rival you meet at the beginning of the game. He's not someone you've known before, per se, but he does end up becoming the champion and earning all the badges.