Miki Kaoru is the sort of person from whom great things might reasonably be expected. Brilliant (he's already taking college-level courses), a talented fencer (he's even beaten team captain Juri on occasion) and musician (a piece he composed as a child is famous), a member of the prestigious Ohtori Student Council, polite, well-spoken, good-looking, and coming from a rich family, he seemingly has it all. His parents are separated, and he lives with only his twin sister Kozue in a home on or near the Ohtori grounds. He attends his classes, his fencing practice and his student council meetings, and accepts the admiration and sometimes adulation of teachers and fellow students with a blush and a self-effacing attitude. And he's only thirteen years old.
When they were both children, Miki and his twin sister were nearly inseparable, and Miki loved to play the piano with her and considered them both geniuses on the same level. However, his memories of the time are heavily idealised. In fact, Kozue's skill was nowhere near that of her brother, but his was so great that he covered for it without anyone noticing. At one time, Miki was too sick to perform, and Kozue was forced to go out alone. She ran away without even attempting to play, and since then has refused to ever play piano with Miki again. That was what Miki considers the end of their idyllic youth, though he has never quite understood why it happened.
Miki is, generally speaking, just as nice and polite and well-behaved as he comes off as being. However, there is an undercurrent of frustration to his behaviour. Much of it comes from Kozue, who over the years has grown more and more obsessed with him, while simultaneously rejecting the sort of person he is and expects her to be. She deliberately acts out in ways Miki disapproves of (often by dallying with boys Miki dislikes) in order to get his attention. As they live alone, Miki feels obliged to take care of Kozue, but is hampered by his inability to control her actions and his stubborn refusal to understand why she isn't, and can never be, the idealised perfect sister he believes he had in his youth. In fact, Kozue actually considers it her responsibility to look after him, as she (not without cause) considers Miki naive and prone to be taken advantage of by unscrupulous peers and teachers. Miki never ends up quite knowing what she does behind his back to preserve his innocence, however.
Miki is also confounded somewhat by the longings that puberty is beginning to instill in him, a situation not helped much by the heavy hints Kozue keeps dropping that she'd like to be the one to satisfy them. He also has a deep-rooted distrust in adults stemming from his own parents, and how they both split apart and left him and Kozue to raise themselves. While he is polite to them (Kozue isn't even that) and defends them if pressed, he doesn't look to them for guidance, and also gets his back up if he feels he's being selfishly ordered around by other adults.
Miki quickly developed quite a crush on Anthy Himemiya, the Rose Bride, whom he projected as being like the idealised version of Kozue he remembers, rather than the debased version he sees now. His infatuation with this idea is so strong that he has begun remembering the Kozue of the past as looking somewhat like Anthy, despite the fact she did not. That being said, Miki's intent towards Anthy is still essentially pure, and when he duelled Utena for her hand, he did so in the belief that Anthy wanted to be free from her. This delusion was dashed when Anthy in the middle of the duel cheered for Utena over him, which caused his resolve to falter and thus he lost. Despite this disappointment, he has remained friends with Utena, and still very tentatively pursues a relationship with Anthy.
Miki is an accomplished fencer and can thus be safely considered to be in quite good shape. He fights with an epee, and is a world-class talent for his age. However, while he can challenge Juri and has far more skill than Utena or most others in his age group, his physical ability shows no indications of being superhuman, and he is unlikely to be able to defend himself much better than any other boy of thirteen.
As a final quirk, Miki habitually carries around a stopwatch, and often uses it to time seemingly random things. Why this is is not known.