Falling in Love with a Sick Girlfriend Twenty Years Younger Than Me

Dumped by my girlfriend of five years, I walked down the streets of Tokyo in despair.
At a corner, a faint cry for help reached my ears.Involuntarily rushed in.There, a beautiful girl who was being bullied was looking at me with pleading eyes.
The moment I rushed out.My story with her begins.Asuna: "Uncle." Me: "Huh?"
Asuna: "Okay, when I graduate... Hey, don't date other girls, or I'll kill you."
Me: "... um."