From the Super Mario Wiki
The Jellyfish Sisters, Gigi and Merri, are the owners of a relaxation center of Gwarhar Lagoon in Mario & Luigi: Superstar Saga. When Mario and Luigi arrive at their relaxation center, the Jellyfish Sisters give them hand massages, and possess the powers of the Firebrand and Thunderhand. Gigi wears red while Merri wears green.
After Mario and Luigi learned their Firebrand and Thunderhand ability, the sisters teach them new moves to use with their techniques, such as being able to walk sideways or run quickly. The sisters then ask the Bros to find the twin Pearl Beans, situated in the cave they are in, as payment for the lesson, or pay a hefty price of 100000 Beanbean Coins.
The Jellyfish Sisters are also able to fly or at least glide, as they descend from the sky when they appear to the Mario Brothers.
Two children of their species can also be found; a young girl with a red bow and dress acts as a receptionist in their relaxation center, while a young boy in red swimming trunk indirectly teaches Mario and Luigi how to surf.
In Mario & Luigi: Superstar Saga + Bowser's Minions, their faces are blackish-blue instead of bright purple. In Minion Quest: The Search for Lord Bowser, another child Jellyfish is seen, a girl with a purple bow and dress, who acts as an announcer and judge for Wendy's "Cutie Battle Contest." However, the sisters themselves do not appear in this mode.
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