Spangle appears to be a small starfish that wears shades and carries around a white ukulele.
Spangle is found on Gwarhar Lagoon hidden behind some boulders. Mario and Luigi can only get to Spangle after having the Hammerhead Bros. upgrade their hammers for the second and final time. Spangle then tells the Mario and Luigi that he wants a nice stage to preform on and is happy to go along with them. Mario and Luigi takes Spangle to Hermie III where the both of them are excited. Hermie III then gives Mario and Luigi a badge.
It should be noted that all of this is completely optional. However, if Mario and Luigi do get Spangle to Hermie III, then Spangle can be seen during the end credits on top of Hermie III where the two of them are waving to the Mario Bros. as they travel back to the Mushroom Kingdom.