A Small Lion

The story starts with a cat, Futenyan, getting stuck in a fence and injured. A high school girl on her way to school with friends (Hibiki) notices him, takes him out, and puts a bandana on him to bandage up his leg injury before leaving. He longs to show her his gratitude and love her the way a human male would after experiencing her kindess, and sheds tears over his inability to do so. Suddenly, two magicians (called "The Moon" and "The Sun") approach him; taking pity on the cat, they turn him into a teenage boy (Minami) who attends the girl's high school. Although he is now fully human, he still has some of his cat mannerisms. For a time, the cat is able to interact with the girl, and she develops feelings for him. However, the magicians warn the cat that he can't do this forever: they'll revoke their magic if he ever tells her the truth about his transformation, and everyone's memories of him in his human form will disappear. Even with this knowledge, the cat gathers his courage to confess and races to her before time runs out. He barely gets the first words out before his human form finally disappears, and he reverts. However, he still interacts with the girl as a cat, and she still treats him with as much affection as she did before his initial transformation. (Source: Confession Executive Committee Wiki)