Sawatari Michiru

A kid that appeared after Rei failed to completely break his clock during a encounter with his sister. She loves the protagonist, and competes with Michiru for his attention.
Although the protagonist concluded that she is her sister from the past thanks to Cro (Kuro) "explanation",

Spoiler

she is actually Rei and Michiru's daughter from the future, using her mother name as an alias.

After introductions were made, they decided to take care of her until she could return to her own timeline.