By winning trophies with a character, he will climb in rank. It’s a system to establish your high scores with each character, because even if you lose, the highest rank you have reached will be kept.
Your characters start at rank 1, and the highest is rank 35, reaching 1000 trophies.
Here are the different appearances of the ranks :

Rank 1 : x0 Trophy

Rank 5 : x40 Trophy

Rank 10 : x115 Trophy

Rank 15 : x215 Trophy

Rank 20 : x340 Trophy