Neko no Onee-chan ga Oshiete Kureta Koto

The protagonist is a rich little boy who lives a lonely life: his mother died when he was younger while his father is always on business trips and himself doesn't have any friends at school. Luckily, his sole source of companionship is his pet cat who is always with him, but tragically died during a run over.
A ground keeper tells him an old tale that someone buried a dead dog near a lake, and the dog resurrected the next day. Taking this to heart, the grieving protagonist does the same thing with his cat, but nothing happens.
Thereafter, a mysterious woman named Azuki makes an apparition in his life, as his guardian and surrogate sister.