Mr. Apothecary

 is an old store merchant who makes potions for all witches and warlocks. He chases the witches who "pop in" - mostly Esmeralda and Samantha.

Episodes

 * A Plague on Maurice and Samantha
 * Samantha's Lost Weekend
 * Samantha's Secret Spell
 * Samantha's Witchcraft Blows a Fuse

Trivia
His real name is Postlethwaite, not Mr. Apothecary, as can be seen just before cutting to the inside of the apothecary we see outside a sign posted "Postlethwaite's Potent Potions" in the episode "A Plague on Maurice and Samantha".