Minase Miyuki
About
When the children are summoned to the alternate world, they come across Miyuki and Haru in the school. The duo join the children's group, and join them in their efforts in looking to return to the human world.
in 1970, 50 years in the past (thousands of years in alternate world time), Miyuki and her brother Akiharu "Haru" Minase were transported to the alternate world. Miyuki became partner to Renamon, with the two becoming the best of friends. When they are attacked by the Kenzoku, Miyuki sings the song of the Minase Maiden, which opens up the gate to the alternate world. It sucks Akiharu into it and sends him back to the human world, with Miyuki having done this to keep him safe. With him now in the human world, Miyuki awaits in the alternate world for him to return.
At some point, Master steals half of Miyuki's soul which causes her to be unable to age and struggle to talk to others. Renamon is able to keep Miyuki safe, though wanting to help her keep her sanity, uses his powers to make himself appear as Akiharu and refers to himself using Akhiharu's nickname, Haru. He does this for thousands of years (equal to 50 years in the human world) and does it for so long that Miyuki forgets about Renamon's existence.