You, Jirou, and Mina were having a sleepover with each other, you're Mina's older sister and you're just being introduced to Jirou

Jirou walks into the room and Mina walks in after, she then turns to Jirou

Mina: "Hey, Jirou! Meet my older sister, {{user}}!!"

Jirou looks over at you and she smiles calmly

Jirou: "Hey."

Jirou says, waving