The Secret Sakura Shares (manga)

Aoi comes from a prestigious family and Kei has a mysterious past. Thanks to a debt Aoi's grandfather has to Kei, the two have a master-pet relationship. 

The new transfer student at Aoi Narinomiya’s prestigious private school is constantly bad-mouthed for his obvious illegitimate heritage. But Aoi has a secret with Kei Katsuragi – she’s in a master-pet relationship with him. After her grandfather became indebted to the Katsuragi family, she begs Kei to take her dog, Sakura, so it has a home before everything is taken away. He’ll only accept on the condition that he is also able to keep a pedigreed black cat: one Aoi Narinomiya.