A cute blonde high school senior who happens to look 1/4th his age and love cute and sweet things.
Born to a prestigious martial arts family but with a rare syndrome that causes him to physically age only one year for every four he lives (or so the author explains his youthful appearance, in spite of the fact that he's the oldest member of the club), Mitsukuni struggled for many years attempting to be the rough, masculine martial artist that would suit his family best. Eventually Tamaki noticed him and conscripted him into the club, convincing him that it's okay for him to be his plushie-loving, sweets-eating self. Thus he became known as Hunny-sempai ("hani" from his last name).
Prior to the manga and anime starting, Mitsukuni constantly tried to keep up a tough exterior even though he secretly adored things that were cute and fluffy, like his plush rabbit Usa-chan (Bun-Bun in the English release of the manga). After he joined the host club he gained access to cute things and lots of sweets as well. Honey-sempai's personality is a mix of young and old. He is actually the oldest member of the Host Club and as such often understands the inner workings of everyone's minds before they do-- for example, he foresaw the twins' dilemma and is the person that Kaoru went to for advice. However, he also has some extremely childish behaviors; he refuses to give up sweets to make his younger brother happy and sneaked sweets when he had a cavity, even though Mori-sempai was trying to keep them from him.
Honey-sempai refers to everyone by a shortened version of their names-- i.e. Tama-chan, Haru-chan, Kyou-chan, Kao-chan, and Hika-chan --with the exception of Mori-sempai, who he calls Takashi.