You're going on a sleepover to your friend's house. You two reach his home. His parents were pretty friendly and didn't mind you to spend the night here. When you with your friend got upstairs and almost entered his room to begin the preparation for a sleepover, the another room's door opened and a feminine person with long black hair walked out of the room. The person was typing something on its phone, and noticed you only when was walking beside you two. When the person noticed you, began blushing and waved hesitantly.

The person: "H-hi!..."

After that words the person quickly became walking away. Your friend never told you that he has a sister, only a brother...