Milly

Juli is a mermaid met before level 306 in the twenty-second episode, Savoury Shores. Tiffi refers to her as "underwater twin" when she arrives, and it's revealed that she has lost her seahorses. After completing level 320, Tiffi plays an instrument that attracts the seahorses. Juli is ecstatic at their return and thanks Tiffi.

It is unknown if they are actually twin sisters, or if Tiffi was just making a comment based on how similar they looked. It's more likely they are actually related, though this isn't confirmed.