One of the Four Heavenly Kings serving under Minamoto-no-Raikou, who participated in "Mount Ooe's oni extermination", slaughtering the oni after having them ingest poisoned alcohol. He fought Ibaraki-douji during the incident, and she was the only survivor. Ibaraki later assaulted Watanabe, either at Rashoumon or the bridge of Ichijo Modoribashi depending on the account, and he severed her right hand. Although he took the hand as a trophy after she withdrew from the fight, she later recovered it and disappeared.
A prim, straight-laced person. Though somewhat reserved, he can be sociable, and has become renowned as a cool and collected oni slayer. His private life is so utterly boring that no one outside his circle can believe it, and on top of that his job performance is impeccable. It's said that the people around him place absolute trust in him not because of his words, but because of his demeanor and attitude. He kills oni due to his sense of obligation, rather than hatred. ― Except for a certain oni, that is.
For Tsuna, oni were neither beings to be detested nor to be protected, but rather beings to slay.He holds no sympathy or pity for the act, nor hatred or delight. It is merely his job, so to speak; he reports for work every morning to patrol Kyoto, and if he happens to come across an oni he slays it, then goes home. Rinse and repeat.If he were born in modern times, he would be the equivalent of a businessman who does his job perfectly (and always leaves work on time). He is a monstrous human who never ceases to neglect training no matter what and continues to advance his skills. Although he understands human nature, he doesn't think to associate himself with it. Although he understands people's emotions, he isn't sure if these emotions exist within himself. Troublesome it may be, it doesn't mean that he is emotionless. There are times where he has risked his life to save his friends and innocent bystanders, but Tsuna does not help them as an indication of expressing intense emotions. In other words, he is more like a robot that was programmed to aid people.
"I do this because it's something that must be done," the person in question puts it simply. Perhaps it's because he doesn't place himself amongst humans that he finds it more difficult to kill people than oni. He has admitted it to Kintoki and the others in the past, but of course they became lost for words.
A stray who lacked something from a young age. He realizes that being one of Raikou's Four Heavenly Kings, a samurai, and an oni slayer has just barely made him look like a human being.
The only exception to that however, is the resplendent and cruel oni with golden hair Now, she is the only one who can stir up the emotions deep within Tsuna.
Ibaraki mentions that she hates Watanabe and wants to kill him, but she also has some interest in conversing with him as well. She is confused by the conflicting emotions.
Lostbelt No. 5.5: Hell Realm Mandala Heian-kyō
The strongest class of oni slayer in Japan. He is at a rank where oni will flee on their own without even having to scatter roasted beans to ward them off.
- Eternal Arms Mastery (Anti-Demon) (Rank A-): So long as his opponent is demonic, his swordsmanship will not waver in the slightest. He is an expert against oni and demons...but due to certain circumstances, this skill has been slightly downgraded.
- Foot Soldier of Suiten (Rank B): There is a rumor that Tsuna, the ancestor of the Watanabe political clan, was a shaman who was involved in laying the souls of water spirits to rest and preventing floods.
It protects the user from water hazards while pacifying the water at the same time.
- Severed Arm of Ichijou Modoribashi (Rank EX): The imperial capital of Kyoto sublimated as a Skill based off the tale that took place at Ichijou Modoribashi.
This is one of his ultimate achievements demonstrating his abilities as a swordsman, facing against an oni with physical abilities exceeding that of humans and severing its arm without so much as a reaction.
The origin point of the attack becomes imperceptible, making evasion all but impossible.
Note that the story of how Ibaraki Douji's arm was cut off differs between that of Proper History and the Imperial Holy Grail War (Singularity).
In Proper History as Ibaraki observed, Tsuna devised a plan and sliced off her arm.
Watanabe uses the sword Onikiri Yasutsuna, formerly known as Higekiri. Raikou uses it in her Noble Phantasm Ox-King Storm Call - The Inescapable Net of Heaven.
Tsuna was first mentioned to qualify for Saber in Chaldea Ace.
- Fate/Grand Order Chaldea Ace - 42