Secret boxers:

In Open Style, fight until you are champion to
access a Jack-In-The-Box boxer. Defend the
championship title five times to access Snake. 

In Peek-A-Boo, fight until you are champion to
access Kiki and Mimi. Defend the championship 
title five times to access Edward King (the 
promoter). 

In Peek-A-Boo, fight until you are champion to
access Roboxer. Defend the championship title 
five times to access Carrie (the girl that 
carries the round cards).