Mitsukuni is a very potent martial arts master, even to the degree of becoming known as the "dreadnaut" of his already very well respected family's dojo. There have been claims that he single handedly dispatched a large number of well trained United States marines and even that the Japanese Defence Minister personally asked him to hide his true fighting potential so as not to alarm other countries and avoid a possible war.
He is at almost all times with his friend Mori who's quiet, and reserved introspection contrasts with Hunny's sweet, childlike nature and works well to impress the Host Club's female customers.
He has an extreme sweet tooth and spends quite some time eating candies and cakes. Upon forgetting to brush his teeth twice (something that Mori usually reminds him to do) he developed a cavity which caused him great pain and led to the Host Club banning sweets of all kinds until he got better. This development led to great turmoil in Hunny's life and his so called "3 days of hell" in which he was constantly trying to trick people into providing candy for him and even at the end to eventually assaulting Mori. The two quickly made up once Mitsukuni came to realise that he'd gone too far and their friendship just became stronger.
As with the other members of the Host Club, Renge the Host Club's self appointed manager, suggested that he "improve" his appeal with the club's customers but becoming a secret bully, though this didn't last for very long as with the other suggested "improvements" it was out of character.