Liquus is an eel-like creature that Princess Nautilus summons when she is in danger. Liquus has the ability to shock opponents like an electric eel, and is a very fast swimmer. Liquus is summoned through the Royal Instrument of Summoning.
It is implied that Liquus assists every Ripple Kingdom princess, although there may be multiple creatures of his kind who are assigned to different Ripple Kingdom princesses.
King Kelp also knows how to summon Liquus, as he taught the skill to his daughter.