Aluka returned home from Meteor Street and was robbed and killed by a person with time and space abilities. He accidentally traveled to the world of Legend of Galaxy Heroes. The time and space transition was such a huge impact that he temporarily lost his memory.
The story begins here.
Statement: Aluka is actually only two years younger than Laiji. Because of his ability, he looks very young, so he is five years younger than Laiji in the household registration.