Maui Mallard in Cold Shadow

Maui Mallard is a "medium-boiled" detective visiting a tropical island when the mysterious Shabuhm Shabuhm idol goes missing.

Besides typical platform game gameplay (running around, jumping from platform to platform), one of the game's most distinctive gameplay features is allowing to switch the player character's form to suit one's needs. The player begins the game as Maui, whose only mean of self-defence is an insect-launching pistol that can launch several forms of bugs, some of them combined for greater effect. However, once the player reaches the second level, Maui transforms into Cold Shadow, his ninja alter ego, who defends himself with short-range attacks using a staff.

Source: Wikipedia, "Maui Mallard in Cold Shadow," available under the CC-BY-SA License.