Harrison Welles is an immortal warlock and is also the Beauchamps' long time lawyer. It is possible that when the Beauchamps were cast out of Asgard, Harrison was one of the witches that was with them.
Harrison is seen as Joanna Beauchamp's lawyer when she is accused of murdering Bill Thatcher. He reveals that though he is immortal, he does not have the ability to cast spells, hinting that he is either not a witch or a powerless one. It is hinted he has a sexual attraction towards Joanna as he tries on multiple occasions to flirt with her. After he won the trial and Joanna was freed, they have a party and end up sleeping with each other. He then leaves.
Harrison Welles possesses immortality but stated to Joanna that he lacks the ability to cast spells.