Arthur is Endora's younger brother and the practical joker of the family.
I quite like him, as he always seems to annoy Endora.
He has been in only ten episodes.

S2E05 "The Joker Is a Card"
S3E06 "Endora Moves in for a Spell"
S3E07 "Twitch or Treat"
S4E31 "The No-Harm Charm"
S5E07 "Samantha's French Pastry"
S5E10 "Samantha Loses Her Voice"
S5E25 "Samantha's Power Failure"
S6E08 "A Bunny for Tabitha"
S6E20 "Super Arthur"
S7E18 "The House That Uncle Arthur Built"

Which is your favorite?

On a side note: Actor Paul Lynde (Uncle Arthur) also played the nervous driving instructor Harold Harold , a normal mortal human, in S1E26 "Driving is the Only Way to Fly"