Flounder

Flounder is Ariel's best friend.

Kingdom Hearts
When Sora, Donald and Goofy first arrive, he was chased by Heartless with Ariel and Sebastian, but were saved by the trio. Because Sora and the others were new to the sea world, they did not know how to swim flexibly. Flounder, then became their instructor and had them chase after him so he could teach them.

Kingdom Hearts II
When Sora, Donald and Goofy revisited them, Ariel was under deep depression. Flounder wanted to cheer her up, and so showed them a statue that resembled Prince Eric and asked them to move it to Ariel's cave.

When Ariel was turned into a human by sea witch Ursula, he and the others stayed near the princess to assist her.