Basara Toujou: Basara was originally Zest's enemy, even though they were enemies she wanted a master like him which made her wish if her creator(Zolgear) was young so she could have a relation like that of Basara and his servants. That changed when she was saved from her own master after she was declared incompetent. She continued to act cold and call him by full-name basis, but later she affirmed that she wanted to serve Basara after he saved her for the second time. She is shown to be explicitly loyal to Basara after her contract completion and wishes to be useful for him. She also harbors romantic feelings for Basara.
Zolgear: Zolgear was Zest's creator and her original master, she was engineered in a way that Zolgear would never touch her. Zest longed for Zolgear once, until she was cast aside by him as he wasn't pleased that Zest once wondered how things would be if she'd have a different master (which was Basara). As Zolgear's aide, she was known to be strong, fierce and most loyal to Zolgear amongst the factions.
Shella: Shella was Zest's temporary master. Zest has some attachment towards Shella, perhaps out of guilt as she was responsible for Shella's capture which lead to Maria's betrayal. Shela was the one who taught Zest the skills of a maid, which became her new occupation while she was under Shella.