Warren is a small mutant earthworm with fuchsia skin and pink and purple stripes, as well as arms. He has blonde hair brushed upright into a pompadour, yellow-green eyes with red pupils and thick black eyebrows. Warren also wears a dark blue dress coat with a yellow-green button.
Warren is a narcissist who has been seen kissing his own reflection and admiring a bust of himself. As such, he believes himself to be the Turtles' greatest nemesis, and thinks that he is more intimidating and noteworthy than he is. Warren also gets fed up when people do not know who he is.
Warren was previously a seemingly-successful news anchor. At some point he was stung by an oozesquito, transforming into a worm.
He followed the Turtles during their initial adventures, declaring himself their greatest foe despite their lack of awareness of his existence. While they were attempting to trap Hypno-Potamus, he entrapped Raph in his "goop trap," which the Turtles found more annoying than frightening. He attacked Leo, who panicked and sliced him in half.
After regenerating, he purchased a blue gem that enhanced his physique from a shop owned by Clem and lured the Turtles to a TV studio to attack them. However, he was interrupted by the arrival of Hypno-Potamus, who cut him in half again. The Turtles ignored him completely in the resulting fight, but when Hypno-Potamus was about to kill Michelangelo, Warren intervened, declaring that he would allow no one else to kill his enemies. In so doing, he sacrificed the blue gem and was reduced back to a small worm, whom the Turtles still did not recognize.
He reappeared in the museum in "Stuck on You," attempting to take the Turtles by surprise; however, they were too distracted by the Foot Clan, Mutant Silverfish and Hypno-Potamus to pay attention to him. Throughout the fight, Warren has been either crushed or used as rope to retie the four Turtles together. At the end of the conflict, he gained a powerful gauntlet, which was almost taken by the Foot Clan, which gave him the ability to fire energy blasts.
He is able to slowly regenerate if any part of him is cut off. The process is extremely painful.
- Donatello: His self-proclaimed nemesis. The only one of the four turtles who remembers anything about him, but usually does not appear to care enough to comment on it.
- Baron Draxum: Draxum didn't know who Warren was at first but was impressed by the power of his mystic gauntlet, which Warren named Charlotte. Draxum recruited Warren into the Evil League of Mutants, but the two have been adversaries since Draxum stole Charlotte (since it was actually a piece of the Shredder's Kuroi Yōroi) and tried to execute Warren.
- Leonardo: His self-proclaimed nemesis. Leo also sliced him in half out of sheer panic, but later did not remember doing so (or who Warren was).
- April O'Neil: Warren and April share a friendly enmity. They are adversaries, but April is also the vice president and longtime member of the Stoneheads, Warren Stone's fan club, and still likes him.
- Michelangelo: His self-proclaimed nemesis. Mikey usually forgets everything about him, though.
- Raphael: His self-proclaimed nemesis. Raph usually forgets everything about him, though.
- Hypno-Potamus: Warren interfered with Hypno-Potamus' attempt to kill the turtles but the two antagonists became allies when they both joined the Evil League of Mutants. By the episode "Warren and Hypno, Sitting in a Tree", the two are revealed to have become friends, moved in together and fallen in love. They characterize their relationship as "friends/roomies/magical assistants," but are extremely tactile and affectionate with each other, very visibly treating each other like boyfriends.