Mills is a middle-aged human described as a large man with a muscular, slightly fat build. He is a brown-skinned man of Japanese descent sometimes read as mixed-race. He has a scar on the left side of his mouth and an X-shaped scar on the right side of his chest near his shoulder. A distinctive irezumi style tattoo covers his left arm down to the forearm and part of his chest and back. Its style and imagery suggest a yakuza background in conjunction with his scars. It depicts a red dragon descending his arm through blue waves that crash into white seafoam below his elbow. He has had a strong fondness for this design apparently since childhood and it is incorporated in some way in all of his appearances.
He is generally depicted with very short graying dark blue hair in a spiked crew cut with sideburns and patches of stubble. In more recent appearances he is clean-shaven and has shorter hair with tighter edges. His monolid eyes are an icy blue and he has thick, expressive brows. He has a small silver hoop earring in his right ear. To his embarrassment, he blushes easily and often has a difficult time maintaining eye contact in romantic situations.
Noted for "dressing rather oddly for the professional environment" in his original appearance, bare skin, traditional Japanese clothing, and the pattern of the red dragon and waves are recurring elements of his design.
Mills has an outstanding dedication to his job as principal and maintains a stoic and intimidating if unusual image, once even mistaken for a supervillain by a student at Champion Halls. At school he can often be seen with a stern expression and some sort of improvised weapon over his shoulder. While he only does this to keep up appearances, he has a no-nonsense attitude to match, and his policies reflect that.
He places a strong focus on discipline, both on a student's duty to deal with the consequences of their actions and on their ability to use their strengths to remedy them. This is best seen during his time at Champion Halls, where students in detention have to directly address their reasons for being there, often using their powers. Nevertheless, his tough methodology appears to keep his students in line and he is ultimately respected as the head of the school.
While he might come off as a tough guy at first, he is a hopeless romantic at heart and thoughtful, if shy and tense, in romance. He was once married to the love of his life, and though he's unsure if he'll ever feel that way again, he desperately wants to.
- No one knows whether Mills is his first name or last.
- He is left-handed.
- In Arcana Break his tattoo depicts a shisa rather than a dragon, in reference to the lion typically depicted on the card he represents, Strength.
- He wears a high school kendo club jacket, but it is unclear whether it's from his own high school years or if he is currently a kendo club instructor.
- He enjoys ramen, fresh pasta, and pastries.
- He keeps framed photos of his dates in his office.
- He is featured as a playable character in BUTTON SMASH!, the fictional fighting game in Game Night Fight!
- ↑ As evidenced by his appearances in the Love Charm series, Mills is bisexual. It should also be noted that the gender of his ex-partner is left ambiguous.