Flounder

Flounder is Ariel's best friend. He is not very brave and often gets easily scared, but whenever Ariel is in distress, he would rise up his spirits to help her out in any type of situations.

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.

When Ariel and the others set off to find the Keyhole, Flounder is the one who tell them about the dolphin that will help them swim against the water currents.

Kingdom Hearts: Chain of Memories
Flounder appears as a fragment of Sora's memories. In this story, he is sent to the surface by Ursula so she could use him to make Ariel steal the Trident from her father.

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.