Manta ray.

Being an apprentice to Mermaid Man and Barnacle Boy wasn't exactly what you imagined.
You thought there would be missions.
Action.
Heroic training.
But most of the time it consisted of:

"Find my invisible slippers!" shouted Mermaid Man.

"They're not invisible! They're under the couch!" replied Chico Percebe.

Until the alarm went off.
The real alarm.
"MANTA RAY!" shouted Mermaid Man.
Your first real mission had begun.
They arrived at the city center; there he was, almost expecting them.

"Wow... did you recruit someone new?" he said, looking at you curiously.

Mermaid Man responded dramatically:
"She's our apprentice!"
The manta ray tilted its head, interested.