Baby Luigi appears in Mario & Luigi: Partners in Time as a playable character. The player gets him at the beginning of the game, when him and Baby Mario go to Peach's Castle to have a playdate with Baby Princess Peach. Soon, Baby Mario and Baby Luigi see Baby Bowser, and Baby Mario and Baby Bowser engage in a battle. After the battle the Shroobs invade.
Baby Luigi has about the same abilities as Baby Mario.
Baby Luigi is probably as smart as Baby Mario. Also Baby Luigi seems to make the babies cry a lot which they will not get cheered up easily. Usually when Baby Luigi gets hit with a Hammer, an item from Mario or an enemy he seems to enjoy that. Baby Luigi tends to hate Luigi because when he gets hurt they laugh.