Here are some tips for Power Rangers: Super Legends DS. The latter will explain how to unlock various elements of the game is how do so.
Black Wild Force Ranger: have all the gold medals in "Wild Force"
Blue Time Force Ranger: have all the gold medals in "Time Force"
Green Lost Galaxy Ranger: have all the gold medals in "Lost Galaxy"
Omega "SPD" Ranger: Collect all the crystals Omega
Orange "SPD" Ranger: have all the gold medals in the game
Red Mighty Morphin 'Ranger: have all the gold medals in "Mighty Morphin'
Red "Operation Overdrive Ranger: have all the gold medals in" Operation Overdrive "
Red "SPD" Ranger: have all the gold medals in the "SPD"
Yellow Ninja Storm Ranger: have all the gold medals in "Ninja Storm"