The relationship between Coolie, Tailor and Donut.
After Sing has defeated the Beast with the Buddhist Palm, the Beast asks Sing, "How did you do that?" to which Sing answers, "I will teach you." The Beast breaks into tears, drops to his knees, and addresses Sing as "Master."
Another example from the end: Now that Sing has offically defeated the Beast and become the One, making him the most powerful martial artist in the world, what does he do? Become a hermit? Travel the world seeking worthy opponents? No. He opens a candy shop. With the girl from his past. His final scene in the movie is the two of them, symbollically returned to childhood, running into the store together hand in hand.