The reincarnation of the bandit who was Ultimo's previous master. Yamato found Ultimo in a Antique shop. At first he didn't want to get involved with Ultimo but eventually does so to protect his friends from harm.
Agari Yamato is an abnormally tall young man, and the son of the beautiful Agari Fushimi. He attends Senjo academy with his best friends Kodaira Rune, Oume Hibari, Otake Akitsu, Matsumoto Kiyose, and the object of his affections (and often, his obsessions) Sayama Makoto. Despite his appearance, he is very kind towards others, although he often makes questionable choices. In a past life, he was an infamous bandit of the same name, believing that the nobility was the root of all evil. That period in time-- the 12th century-- was when Yamato first met Ultimo, Vice, and their mysterious creator Dr. Dunstan. At first believing they were live human children being locked in boxes, Yamato threatened Dunstan, who was far from scared or even bothered. Vice then wakes up, annoyed by the racket Yamato had been making. Upset, Vice begins to kill Yamato's comrades (some of which resemble his aquaintances of the 21st century), until Yamato attempts to fight Vice; Ultimo then steps in. While it is not shown, it is said that Vice was momentarily defeated, his whereabouts thereafter are unknown. Ultimo then decides to accompany Yamato and the bandits, and it is implied that they completed the pledge of master and page, causing the events of the current day.
Now in the 21st century as a young high school student, Yamato has no memories of his past life or Ultimo, and is visiting an antique shop with his best friend Rune, to find a gift for his long-time crush, Sayama Makoto. It is there that Ultimo is found, asleep in a glass case (after being believed to be just a doll by the shopowner), and Rune and Yamato are puzzled by how real he looks. After watcing the 'doll' for a moment, Yamato vaguely says 'Ultimo'; even he himself doesn't know why he said it. Ultimo awakens, much to everyone's suprise, and breaks through his case to hug Yamato. Yamato is in shock, because not only does he not know this boy, he is covered with broken glass from the case.
Rune and Yamato hurriedly leave in shock, not at all understanding what just happened, unaware that Vice is in front of their bus. He attempts to slice it in half, only to be interrupted by Ultimo; he ends up cutting it unevenly, and no one is harmed. Ultimo appears (having followed them) and attempts to explain, when Vice cuts him off -- literally -- reaching to his spirit sphere in his chest which, if destroyed, would kill him. Boldly (and mindlessly), Yamato jumps out of the bus and attempts to hurt Vice with his schoolbag, claiming he won't let Vice harm Sayama, although she was never in danger. Happy that Yamato is still the same as his past self, Ultimo is able to again momentarily defeat Vice, cutting him in half. Vice's limp body is then retrieved by K, his current master. Later, certain events transpire and Yamato completes the pledge with Ultimo, regaining all memory of his past and future selves.
It is said in an extra that Yamato is so big because his favorite food is milk, and he was teased briefly on his first day of 5th grade by the kids after one points out that milk, in fact, is not a food. It also states that his favorite thing is Sayama.