Seaclya (or Cecilia) is a demon who live in the Cursed Crown. Her objective is to train Lilina in oder to make her the new Sucubus Queen.
Before she become a demon, Seaclya used to be human. She never talked about her previous life (maybe she couldnt remember it). She live in the Cursed Crown, waiting for a new wearer to become Lust Queen. It seems that a previous chosen one were Nastela. We dont know how Nastela died but the Crown were hidden in her tomb. When Seaclya felt Lilina in Anvil, she attracted her to the tomb and tricked her to touch the Crown. Finally, she could control her in order to put the item on her head and started to train her in order to make her the new Lust Queen. Even she said that Lilina's destiny is to become King Inma's wife, Seaclya won't see this day and try to find a way to avoid this destiny to Lilina.
Acording to which end is reached, Seaclya can manage to turn Lilina to lust queen or not. The canon end make her to gain divine power (as she controled Abaddon Parasite like in the true end) and trained well Lilina. in Cursed Armor II flashbacks, we can see that the Human and the Demon share a high complicity : Seaclya can take control of Lilina's body (with her agreement probably) to have fun. She even teached her how to use Sucubus power to undress people and have sex without they noticed it (in Cursed Armor I, this is Seaclya who do this job)
But, in Cursed Armor II, Seaclya were sealed by Beryl. She asked to Lilina to "live good", like she understood that she probably never see her again.
As a demon, Seaclya is a really perverted person and don't really care about Human's business. The only thing who matter to her is to reach her goal : make a new Lust Queen. For this, she can do a lot of things like making deals, manipulate or just force people to do it with some manipulation spell. But she's not totaly a monster : she really have some feelings for Lilina, trying to help her when her Mistress is in trouble or even avoid her to fall under Inma's King power.
Nastela : It seem that Nastela were the previous Lust Queen and Seaclya ex-master. We dont know how Seaclya saw Nastela and how her Mistress died...
Lilina : She is Seaclya's Mistress and future Lust Queen. At the begining, the Witch were reluctant to cooperate. But, with time and some terrible events, she ended by trust the sucubus and cooperate. Finaly, Lilina had some real affection for the sucubus. Seaclya often let Lilina in delicate situations like become Sucubi's seedbed, a sex slave for some people (Owen, Torres citiy's people, etc) but always did her best to put her out of real trouble (like Magical Academy's event or Abaddon parasite case).
Hiller : when she saw him, she understood who he really were. Seaclya wont cooperate with him and did all she can to kept Lilina to surrend to him.
Devil Eye : another demon. Once again, Seaclya avoid to cooperate with him. But she tried to make a deal to prevent Lilina to be broken by Beryl. The monster refused due to the fact it were Beryl's business, not him.
Beryl : Seaclya never loved her. This girl were a real menace for Lilina's sake and she did all she could to save her Mistress when she had to deal with the rapist.
Inma King : No mater if this demon is his Master, Seaclya's loyalty is purely forced. Maybe this is due to her previous human life, but Seaclya's feelings push her to find a way to keep Lilina away from him.
This is the only demon who felt no loyalty to Inma King.