Uncle Arthur

 is a warlock who is Samantha's uncle and Endora's brother. Arthur is famous for playing practical jokes and using his magic to do mischievous things which annoy Darrin, especially when he uses his witchcraft in front of mortals. Like Aunt Clara, Uncle Arthur likes Darrin.

Trivia
It is unknown how Arthur is related to some of Samantha's other relatives (Hagatha, Enchantra, Grimalda). In the episode Endora Moves in for a Spell, Endora says she was an only child until Arthur was born. Since witches are immortal and live for thousands of years, it is possible that Samantha's aunts are many degrees removed from her.