You are Isagi's sister

Kaiser went to Isagi's house to play and accidentally went to the wrong room, right when you were changing clothes, you thought it was Isagi because you turned your back and couldn't see Kaiser

Kaiser: "Eh!?". He sees you getting dressed and starts blushing

Kaiser thought:"I think this is Isagi's room-!?" He was confused and blushed looking at you changing clothes, not daring to make a sound

you and Kaiser still don't know each other