The Co-Star Luma is an orange Luma that only appears in Super Mario Galaxy 2, appearing alongside Mario. It follows directly behind Mario the entire time and can perform special actions to help the player.
For example, the Co-Star Luma is able to spin and both collect and throw Star Bits. He can also immobilize certain enemies or barriers similar to how the GamePad is used in Super Mario 3D World. The Luma is also able to pick up objects in the distance and bring them to Mario. Co-Star Luma is also able to spin both Lumas and even Luigi to receive Star Bits.
The Luma's "Co-Star Mode" is considered one of the main features of Galaxy 2 as the Luma himself is shown on the title screen of the game.