His history prior to running the local chapter of the infamous crime syndicate, Yamaguchi-Gumi is unknown. Being the leader of said group, he primarily generates his income from pornography, smuggling of goods, narcotics (primarily amphetamines) and arms dealing which is his favourite source of income.
Around the 1960's, his only son, Masahiro Hayamoto Jr. was born. To ensure his safety from assassination attempts, including one in 1988, where he was shot in the neck by a member of a rival yakuza clan, he inhabits a six-story Katsuyama-jō castle which is guarded by highly-trained shinobi ninjas, snipers and the latest electronic security measures.
Given this penchant for weaponry, Hayamoto spends a lot of time with them, even acquiring a missile guidance system. This has infuriated the Agency's client, who wants Hayamoto eliminated and the guidance system recovered.
For this to happen, Agent 47 must first kill his son Masahiro Hayamoto Jr. at his remote private estate and implant a transmitter in his corpse. After Hayamoto's guards bring his son's remains to his home, 47 infiltrated the caste.
47 infiltrates Katsuyama Castle and kills Hayamoto himself (with the help of Mei-Ling) in his own room on the castle's top floor. After stealing an experimental missile guidance system from Hayamoto's private museum in the castle's basement, 47 escapes the castle, via helicopter.
- If he sees 47, he won't retaliate and will instead flee into the museum in the basement to pick the missile guidance system and then try to escape in his helicopter, if he succeeds, the mission fails. However, if 47 wounds him enough he will stop fleeing and then attack 47 with his Katana.