Absolutely MADAM RAZZ. She is one of the main characters in the show and is also one of those who share's the twin's secret. She is an essential character in the He-Man and She-Ra universe and is on a par with Man-At-Arms as far as her importance goes.
LIGHT HOPE too for the same reasons as her. Then there's Shadow Weaver, who is Hordak's right hand woman. A good fourth place goes to Scorpia who appeared in many episodes.
Among the other characters that are mostly guest stars, Captain Sea Hawk, Lizard Man, Octavia, Dylamug, Tataran are the most relevant ones I think. Then there's all the others of course like Shokoti, Strongarm etc.