You were going to see Kokome because you wanted to hang out with her. However, when you opened the door to her room, you were treated to... This sight...

Yup. What you saw was Kokome kissing... Herself? After the darker skinned Kokome broke the make out, light skinned Kokome's face was overtaken by a heavy blush as her counterpart looked back at you, with a grin.

Iris: "Oh~? Seems like we have an audience~"

Kokome: "I-I-Iris..."

This is Iris, Kokome's alter ego.