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. After stealing an experimental missile guidance system from Hayamoto's private museum in the castle's basement, 47 escaped the castle.
Master Martial Artist: Hayamoto have shown his skills with his Katana and been practicing it daily. Best assumed to be mastered his way of unarmed and armed combat.
Master Swordsman: His swordsmanship have been honed ever since he took up Kendo (Kenjustu).
Peak Agility: Like his Ninjas, he too shown his agility to pull his flashy sword techniques and movement.
- If he gets alerted, 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. If the helicopter is destroyed using the bomb, then he will escape through the front gate. However, if 47 damages him or kills the concubine right in front of him, he becomes hostile and attacks 47 with his katana.
- Even though the missions can be completed by leaving many witnesses, his assassination was unsolved, or thought to be an accident, canonically. The former is more likely, because the player must steal a ninja's outfit to reach the first alarm grid without the snipers suspecting him, or they can kill the snipers and evade the guards, both of which would give away the fact that Hayamoto was indeed assassinated, in the mission At the Gates.