While he was a student, Okamoto worked for Arc Work Systems on a part-time job basis and participated in the development of various TV/Windows games based on the Sailor Moon series. After graduating, he joined Bandai, and while there, continued to take part in game development, but this time he helped plan puzzle games for Sailor Moon. Although it was his dream occupation since his high school days, Okamoto gave up on it when he graduated from university. He later resigned from Bandai at the age of 28 in order to become a professional manga writer, another dream of his.
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 January 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.
In a July 20, 2015 tweet, Okamoto expressed that he felt hopeless while struggling to become a professional manga writer at the age of 30 after quitting from Bandai. He then learned the writer of the then-famous manga Hokuto no Ken / Fists of Hokuto (1983-1988) had finished the manga at the age of 27, inspiring him to continue with his own manga.
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.
In the interview published in Aug. 2016 in a magazine (Febri. No.36), Okamoto says he originally wanted to write a manga like "Lovehina" , a love comedy, before starting the serialized Elfen Lied. He adds that he was not quite confident if or not he could develop a love story then.