What Halloween character's name comes from a Saxon word meaning wise one?

The Word "wizard" comes from Middle English wys "wise" plus the common ending -ard (cf drunkard, bastard) meaning something along the lines of "one who is" (usually in a pejorative sense). Could that be what you're thinking of?

Another possibility would be "witch" (from wicce, the feminine form of Old English wicca meaning "sorceror, wizard, man who practices witchcraft or magic"). It's less directly from "wise one", but fits the "Halloween" part better.