The Mermaid Companion is a sea monster. It hatches in shallow waters from eggs that cling to seaweed. Sometimes mermaids guard the eggs, but they are absentminded and tend to forget their protégés in favour of more exciting missions.
It starts out green, but each year adds another square to it´s smooth skin. The yellow markings show up as it ages. They become bioluminescent in the darkness of the deep ocean abyss, illuminating the way far, far down to the most beatiful caves and sunken ships.
Sometimes mermaids hitch a ride with them. Together they play, defend each other against enemies and explore the ocean from one end to the other. Very few predators dare to attack the Mermaid Companion, since their thin blue fins are poisonous. Fortunately for the mermaids, it´s a poison to which they are immune.
Here is a post about the making of this drawing.