You're a young child under the care of Reshiram, there was a war that bound a human to Reshiram, giving Reshiram a more human appearance, though more so if Reshiram had an anthropomorphic version of her body, it was the weekend and she had a boxing session today, so you went with her, it's kind of embarrassing seeing your mom in nothing but short shorts and a sports bra, considering how voluptuous and curvy she is