10 Days with My Devil has a large cast, including six story characters and a number of supporting characters.
Your best friend and coworker. She plays a significant role in Haruhito's route.
She is your sister, and plays a fairly key role in Kakeru's route.
He is Makoto's husband.
The daughter of Makoto and Yuusuke - she plays a role in Kakeru's route. Her name is written with the Japanese characters for "angel" and "sound".
Your crush in Shiki's route.
Captain Fujikawa is the former captain of Satoru's team before he was with Kakeru and the others. He is quite an important person to Satoru. Like Kakeru, Captain Fujikawa's special ability is manipulating life force.
He is the kind Prince of the Angel Realm. He and Kakeru know each other and seem to be on friendly terms. In Kakeru's sequel, he is shown to be somewhat flirtatious, as seen when he holds your hand and compliments on your "touching beauty". Rui seemed eager to meet you again and continued to flirt with you despite your relationship with Kakeru. Afterwards, Kakeru warns you about Rui, saying that you never know what he's thinking when he flirts with someone.
Rui's fondness of you doesn't end there. In Rein's sequel, he finds you in his castle, invites you into his room for tea, and tries to teach you how to dance for a party he decided to invite you and Rein to. Rui wishes that people would just be themselves around him, and not be so formal just because he's a prince. Your beauty isn't the only thing that he's interested in; he also likes how you aren't afraid to show your true self around him, and he seems disappointed when you begin to act all polite to him just because of his title. While he is interested you and believes that the both of you could get along quite well, Rui doesn't seem to hold any intention of stealing you away from your fiancé.