When he was in high school, he was told he was recommended by his mother to "test" the Animus, being told it was a game. While testing it, he was put under the care of Saejima and Irene, researchers at the Abstergo campus. After reliving some of Edward Kenway's genetic memories, Seijin received a wound that Edward received in the memories. This resulted in Seijin being sedated and imprisoned by Abstergo.
After awakening in a bedroom made for him, Saejima informed him of his situation, claimed he was an ally and asked him to save his mother. Irene arrived and informed him that he was one of Edward Kenway's descendants, as was his mother, who, she showed, was intubated and unresponsive. Irene and Saejima explained to Seijin that reliving the memories of their ancestors could result in bodily harm and the termination of the subject brain activity, resulting in their death.
Irene explained that Seijin's mother refused to relive the memories of Edward Kenway and induced the catatonic state on herself and without the machines she would die. Irene threatened to cut off his mother's oxygen if he would not comply, forcing him into the Animus to find the Observatory.