She's a tall woman with long blonde hair and purple eyes and wears a white mage outfit with a magic staff.
She's a sadistic lady that holds a huge grudge against the Shimazu family, but focuses her anger towards Sakuya. Instead of killing her immediately, she torments Sakuya by attempting to kill anything and everything closest to her until she's given into despair.
In Chapter 16 of the manga, Isuca reveals herself in front of Shinichiro while he was on a date with Sakuya. She confronts Shinichiro and bites his neck injecting something inside of him only then to be stopped by Sakuya, She evades Sakuya's attack and reveals her name to be Isuca leaving Shinichiro and Sakuya confused.
She then returns in the next chapter after tracking down Shinichiro from the bite on his neck. She attacks the family head Nami Shimazu after putting up a barrier only to reveal that she was a puppet. Sakuya and Suseri show up to protect Shinichiro while he takes Nami's puppet to Nadeshiko. Shinichiro returns with his katana named Yashaou to see Sakuya and Suseri in a bind. He quickly overpowers Isuca after letting the spirit of Yashaou control his body but before he could give the final blow, he stops Yashaou but that was a fatal mistake when Isuca gets enraged and tries to kill him even though she was told not to. Before she could finish him off, her right arm falls off her body and wallows in pain. Nami's puppet returns to kill Isuca but is stopped by a myserious man in black wearing a mask when he buts in, takes Isuca and disappears.
In Chapter 26, Isuca wakes up from a dream and converses with a little girl holding a teddy bear about their goal to destroy the Shimazu family. Isuca then agonizes in pain due to her dreams until the mysterious man, whose name is Ildana, shows up and comforts her. She's then later seen in Chapter 30 conversing with Ildana about his battle against Nami. After the conversation, she becomes more determined to kill Nami and the Shimazu family.
She shows up later on in Chapter 41 in an attempt to kill Sagiri Shimazu, but notices that Sagiri is trying to kill Shinichiro. Sagiri escapes with multiple wounds leaving Isuca with a fatally wounded Shinichiro. Isuca then asks him if he wants her to save him. He slightly nods yes, and she takes him back to the Asahina family household. She then watches Shinichiro as he's healing up in a pod. After he's fully recovered, Isuca then takes him outside to have a conversation with him. She then offers him to betray the Shimazu and become hers offering to treat him right, but Shinichiro politely declines her offer. This enrages Isuca leading to her claiming that he prefers Sakuya calling her a chibi. She then climbs on top of him and tries to seduce him, but that was interrupted by the little girl from Chapter 26, whose name is Lilia. Lilia then tries to kill Shinichiro but is quickly stopped by Isuca. Isuca then pushes Shinichiro towards Lilia forcing him to kiss her. She then covers Shinichiro's mouth telling Lilia that he he has the Eye of Truth and has her true name in the palm of her hand. Lilia had no choice but to leave after that. After the confrontation, Isuca falls to the ground after using up some much spiritual energy. She then asks Shinichiro to kiss her and give her spiritual energy. Shinichiro hesitates, but Isuca jumps up and kisses him quickly rejuvenating her spiritual energy. Ildana shows up wanting to speak with Shinichiro leaving Isuca alone. After a while, Sakuya and Tamako show up to find and bring Shinichiro back home. After leaving the household, Isuca pops up asking Shinichiro if he's doing alright. He says yes and thanks her for saving him. She then kisses him again in front of Sakuya and Tamako, and turns to Sakuya telling her that she will take Shinichiro away from her but won't do it today out of consideration for Ildana and walks away.