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, attending 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. Although a bit of an idiot, despite his appearance, he is very kind towards others. In a past life, he was an infamous bandit of the same name, believing that 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 Dr. Dunstan. At first believing they were live human children after the awakening of Ultimo, Yamato threatened Dunstan, who seemed at the least even bothered. Vice then awaken next to Ultimo, annoyed by the racket that woke him up. Upset, Vice begins to kill Yamato's comrades (while some vaguely resemble those of the 21st century), until Yamato attempts to fight Vice, and Ultimo steps in. While it is not shown, it is said that Vice was momentarily defeated (his whereabouts then 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, 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 owner of the shop), and Rune and Yamato are puzzeld by how real he looks. Yamato vaguely says 'Ultimo', and even he himself doesn't know why he said it. Ultimo awakens, much to everyones 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 glass broken by Ultimo.
Rune and Yamato leave in shock, not at all understand what just happened, unaware that Vice is in front of their bus. He attempts to slice in half, only to be interrupted by Ultimo, and cuts it unevenly, and no one is harmed. Ultimo had followed Yamato, and as he explains, Vice cuts through him, reaching to his spirit sphere, 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 is then retrieved by K, his current master. Later, current 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 says his favorite thing is Sayama.