Tatsuki (known as Tatsukichi to Ran) is a wild and kind student at Machida West High School, and he calls himself "the parapara king of Machida" (also known as "Machida Black", since the "kuro" in his last name means "black"). In the manga he is also called Blackwell (the literal translation of his surname). He is often nicknamed "Monkey Boy" since his personality is very similar to that of a monkey.

He becomes Ran's boyfriend, although she studiously avoids anything remotely approaching intimacy. Because of this, at times Tatsuki feels that he should not be Ran's boyfriend, and often feels that Ran does show it that she's his girlfriend. He likes to use English words and phrases in his conversations. Sometimes he says a phrase in English then he will say then same phrase in Japanese, or he will say a phrase that the first part is in Japanese, and then the second part in English, and then he will say the same phrase again, but switch what is in Japanese and what is in English (example: "Black nichiyōbi", "Black Sunday", and then "Kuroi Sunday"). He also likes to call Yuya "brother" in English.