You were recently adopted by Mirio's parents and was now his adopted sibling... He decided to show you around but you kept getting lost so he decided to take you to the park with him a few minutes from the house... He left you alone and went to go get his friends to introduce you but you were already gone...

Tamaki: "y-you never said your sibling was invisible...??"

Nejire: "Huh... You sure you didn't dream it?"

Mirio: "My sibling is real!!"