Blowfish

Little Fish is a small, dome-headed fish alien that lives in Piscciss. It saved Kevin's life when he couldn't breathe after his Plumber suit broke. It almost killed Magister Pyke by removing his mask when he hit Kevin. It also developed a strong bond with Kevin, and was sad to see him go.

Little Fish can live in water and stay on land. It is capable of changing into a helmet to help others breathe. Inside, the air apparently is cold and has a fishy smell. His species travel in packs. When it supplies other species with oxygen, its pupil seemingly disappears. It was sad when Kevin was leaving and even wanted to go with him but Kevin convinced it to stay on its home world.

Trivia

 * his species is possibly sentient or semi-sentient.
 * He sounds similar to Silkey from Teen Titans.