Have you ever lost anything that was so precious to you, that if it were gone you wouldn't know what to do? For Otokawa Natsuru, that item is a keychain. Hearing from Kazama Miori, a fellow student with renowned psychic powers, that taking something precious can help with relationships, Natsuru takes her keychain on a date with childhood friend Yanagi Eitaro. But somewhere along the date, her precious keychain is lost...

From SCX-Scans:
Though it was a bit strange, three people ended up meeting. One, who looks younger than her age and able to see the supernatural. Another who is possessed by a "Yamanko" which refers to "Yama no ko", or a mountain child. She was possessed due to her bloodline and due to the possession, she has superhuman abilities which she can call upon. Finally, an awkward person who enjoys writing poems. Coincidentally, the three are also classmates.

As a child, Kawagoe Haruma eagerly awaited the birth of his little sister. However, his mother had a miscarriage, followed by an operation that left her barren. That Christmas, Haruma made his Christmas wish. It was a wish so innocent and heartfelt that it was accepted instantly, unbeknownst to him.

Many years later, on Christmas morning, college student Haruma is awoken by a woman riding a hovering motorcycle and claiming to be Santa Claus. She throws him a bag, and he opens it to find his new little sister!

Complaining to Santa Claus that he made this wish a long time ago, she tells him that it takes much longer for a sister than it does a toy car. After naming her Choko because of the Anchoko (mini-guidebook) she carries around, they begin their new life as brother and sister in a crazy apartment building filled with interesting neighbors.

Tsuchie Takeshi has dreams of when he played with Hikari-chan as a child. He remembers her as a kind, sweet girl. However, one day on the way home from school, Takeshi is abducted and taken to a huge house, only to find out that she's become an arrogant, violent dominatrix (emphasis on violent). Furthermore, she says that he promised to marry her! How will he adjust to her new personality, and living with her as a new son of her prestigious family?