Kagehira Mika is a second-year student at Yumenosaki Academy, a private high school that specializes in training idols: he is also the other (current) member of Valkyrie, formerly the top unit in the school and a trio until events the previous year ended with their fall and the loss of a member.  He is also Itsuki Shu's loyal and devoted (if disobedient) doll, faithfully staying by his side, tries his best to support him, refuses to let Valkyrie rot away...and spends most of the year refusing to be a person and accepting Shu's harsh treatment of him without complaint.

Absolutely devoted and extremely in love, Mika is willing to go to extreme lengths for Shu's best interests (even if said extreme lengths are not what Shu actually wants him to do), even put his own wellbeing and future at risk. Shu, for his part, spends about as much time taking care of Mika (though with a lot of sharp-tongued complaining) as Mika takes care of him, the two of them co-dependently keeping each other alive.

Much of Mika's arc over the course of the year revolves around his refusal to be a person, rather than a (seemingly) obedient and dependent doll. As an abandoned child who helped/helps take care of other abandoned children in his hometown, he was intimately familiar with how shitty people were, bitterly disillusioned, and decided that being a doll was better than being a person. He was initially confused by Shu treating him better after Halloween, as well as by Shu's efforts to drag him into personhood and independent existence, little by little.

By the end of the year (...after a desperate gambit on Shu's part just before his graduation, meant to force him to work on his own and independently), Mika has taken his first fragile steps into personhood, inherited the Valkyrie name to continue the unit after Shu's graduation and departure for France, and has not dropped out of school to chase after him.  Progress...?