Based on the manga by U-Jin.

This is a story of two siblings, Anzu and Naruto Nozaki, heirs to the fabulous Nozaki fortune. Naruto appears to be an overbearing sis-con. In fact, he's madly in love with Anzu, because they're not blood-related. He confines her to the family mansion for "security," depriving her of companionship and a real life. At age 16, she starts to rebel. She smuggles in a kitten as a pet (Naruto hates cats) and then escapes in a delivery truck. Naruto catches up to her and realizes that she can't be confined like a bird in a cage. He agrees to let have a normal life, but just then, tragedy strikes

(Source: Collectr)