Rupert Giles

Rebelled as a teenager, practiced dark magic, summoned the demon Eyghon
1997: Became Buffy Summers' Watcher, worked as Librarian at Sunnydale HS
1999: Fired from the Watcher Council for defying them to save Buffy, bought the Magic Box
2001: Reinstated as a Watcher when Buffy blackmailed the Watchers Council
2003: Helped create the Slayer Organization after all Potentials were activated
Killed by a Twilight-possessed Angel, resurrected the following year;
Accidentally brought back as a twelve-year-old body, eventually restored