In Championship Mode, get all the gold medals in women's events.  Then, while at the character selection screen, press either R or L.  NOTE: Using a costume will disable you from winning medals and setting records.

Unlock the Bear Costume

In Championship Mode, get all the gold medals in men's events.  Then, while at the character selection screen, press either R or L.  NOTE: Using a costume will disable you from winning medals and setting records.

Unlock the Robot Costume

In Championship Mode, get all the gold medals in men's and women's events.  Then, while at the character selection screen, press either R or L.  NOTE: Using a costume will disable you from winning medals and setting records.