He resigned from Bandai, a toy and TV game software company, at the age of 28 in order to become a professional manga writer. He is known to be an intensely private individual. Residing in Tokyo, Japan, he is the creator, and artist for the Elfen Lied Manga series and was a consultant on its anime adaptation.
He made his debut into the manga world in 2000 with his first short story manga titled "Elfen Lied," which bears almost no connection with his 2002-2005 work, Elfen Lied. He says that the first "Elfen Lied," a tsundere type of love story about young musicians, is the only manga he wrote before becoming professional.
Okamoto recalls in his twitter of July 20, 2015 that he felt hopeless while he was struggling to become a professional manga writer at the age of 30, after quitting the company that he was working for since graduation. He learned at that time that the writer of the then-famous manga Hokuto no Ken / Fists of Hokuto (1983-1988) had finished the manga at the age of 27.
Okamoto's birthday is January 6th, which is known publicly, but does not disclose the year of birth. Based on guesstimation after following his twitter massages for a while, he was most likely born in 1970. He said before that he made a debut at 30 and it was 2000.