"Yellowtail" is the main antagonist. He's the father of Onion, stepfather of Sour Cream, and spouse of Vidalia.
Yellowtail is fair-skinned like his children. He has a light blonde beard and eyebrows, and wears a bright yellow rain coat, with two white stripes near the bottom, a bright yellow rain hat, and pale blue rain boots. In "Onion Friend" a younger version of himself appears in a picture, he has no large beard and appears to have a small tuft of hair similar to Onion.
Yellowtail, like his son Onion, speaks in unintelligible noises, as revealed in "Onion Trade". He cares a lot about Onion, but his job as a sailor leaves him with little time to see him. He may be a somewhat old fashioned father, as he seems to want his step-son Sour Cream to follow in his footsteps as a fisherman, instead of following his dreams of being a DJ. He also likes to capture and eat fish, making him an enemy to The Crystal Seafood.
Yellowtail doesn't see his son often as he appears to be a sailor, but they've shown to have a very good father-son relationship when together, as seen in "Onion Trade".
Yellowtail was shown supporting Greg to help him stand in "Ocean Gem" due to Greg's injured foot, as well as lending him a tissue when he got emotional after seeing Steven be praised by the crowd. However, in "Steven and the Stevens", when Greg warns him that the boat might be too big to fit the car wash, he looks reasonably angry. They may be friends, or simply acquaintances.
Yellowtail is Sour Cream's step-dad, but their relationship appears to be strained due to his disapproval of Sour Cream's choice to become a DJ instead of a fisherman like himself. According to Vidalia, the two fight very often.
Vidalia and Yellowtail are married. However, as Yellowtail is often out at sea, the rest of their relationship is unknown.
Yellowtail and Butt Lobster are mere mortal enemies. Their rivalry began when Butt Lobster purposely shot a harpoon at Yellowtail's boat, with his mother and father aboard, sinking them both.
Minor Characters/Animals is Yellowtail's darklord and God. It gives Yellowtail his power of fishing if in return he hails to it. When Minor Characters/Animals kills his Wife and Children trying to devour the Crystal Seafood, he betrays Minor Characters/Animals, cuasing an emense hatred between the two.
- Yellowtail is named after the fish of the same name, corresponding to the fact that he is a sailor. Together with Vidalia, Onion and Sour Cream, his entire family is named after food.
- In a picture on the wall during "Onion Friend", his younger self looks similar to Clarence, a character in another Cartoon Network show of the same name.
- His job as a fisherman is what makes him an antagonist of Butt Lobster.