In which TV series is Mitch Buchannon a main character?

In which TV series is Screech Powers a main character? Who played Steve Urkel in 'Family Matters'? In which TV series is Steve Urkel a main character? Who played Sam Beckett in 'Quantum Leap'? Who played C.J. Parker in 'Baywatch'? In which TV series is C.J. Parker a main character? Who played Dale Cooper in 'Twin Peaks'? Who played Brenda Walsh in 'Beverly Hills, 90210'?