Not much is known about his past, he used to visit the same school as Tetsu Sendagaya. It is also revealed that he comes from a family of mages. When he was a child, he was once saved by Tsurugi and Touma while he was being chased by vampires. Yumikage's family is apparently very wealthy.
Yumikage has long blond hair tied into a ponytail. He has dark sharp eyes and is tall. He is seen mostly in his C3 uniform, like many other C3 employees. He wears a blue shirt under his jacket.
Yumi mostly seems to be a hard character who doesn't really empathize with others. When he, Tsurugi and Jun first appeared (in the manga) he behaved like the leader of their group - and seemed to be accepted as such.
He seems to be obsessed with the wish to eliminate Tsubaki. Even when Tsurugi is hurt, he still orders him to defeat Tsubaki. After the fight, Yumi says that Jun (who takes care of Tsurugis wounds) spoils him too much.
Although he mostly seems harsh and aggressive, he seems to care for Tsurugi and really doesn't want him to get destroyed by Touma.
Yumi is ashamed about his past - as he was forced to do cross-dressing in order to be a decoy for vampires.
When Yumi threatens someone, he usually threatens with "kind" things like "I'm going to embrace you!". He could also be some kind of Tsundere character. He also has childish attitudes, he has fun in teasing Jun, alongside Tsurugi.
- "This is the end for you..." (Chapter 39, to Tsubaki)
- Yo, long time no see. Same brawn over brains as always, eh, Tetsu?" (Chapter 40, to Tetsu)
- "You know... We are not your 'friends.' This is just our job. Whether we cooperate with you or not is up to the organization." (Chapter 40, to Mahiru)
- "Well, he can't help it. He was bought by a bad old man when he was young." (Chapter 41, about Tsurugi, while drunk)
- "Really? You're Touma's slave, aren't you?" (Chapter 41, to Tsurugi)
- His favourite food is cheese resotto.
- His favourite drink is wine.
- His favourite color is citrus yellow
- When he is drunk, he uses Junichirou to pick him up and carry him into his bed.
- He threatens people with doing nice things (If you do that, I'll hug you!).
- He has been known to cross dress in both his childhood and adulthood.
- Traditionally, Yumi's family dresses boys in feminine clothing until the age of 7 in an attempt to trick demons.
- When he was a child, he had black hair
- Yumikage's code name within C3 is Hodur.