They are introduced in LocoRoco 2 when Majoline kisses a MuiMui, causing it to become red and to develop their new attributes. They act in servitude to the Moja. An example being when they help Majoline revive Bonmucho and then help him fight the LocoRoco.
The BuiBui red body color symbolizes their misbehavior as described above.
Like the MuiMui, the BuiBui have their own theme song called 'BuiBui' which is played as BGM for the BuiBui Fort stages. They also sing a version of 'Moja Song' which is played during the Mutated Bonmucho battle in LocoRoco 2. In Midnight Carnival they sing in new version of the stage songs (aside from 'Dadhi Dado Da' and 'Uwauwauwau'). They also sing in place of the MuiMui in 'MuiMui Crane' which becomes 'BuiBui Crane'. 'LocoRider' which becomes 'LocoBall'. 'Loco Race' which is played in Bonus games and the Practise Area. And even the 'MuiMui Song' which is played at the BuiBui Shop. They also sing the main theme song of LocoRoco Midnight Carnival.