According to Greek Mythology, Thetis was the daughter of Nereus, the old man of the sea, and she had 50 sea-nymph sisters.  The two depicted in the comic today were Erato and Keto. 

Gonj: “What makes they so mean girl?”
Thetis: “Jealousy I suppose.  I was always more developed
then them… and they never forgave me for it.”