- “Oh, maybe you're right. I've seen a part of myself that no man should ever see. But you know what? I like it!”
- ―Evil Ren before splashing himself with XB-49.
He embodies Ren's madness and utter mayhem, along with his desire to hit people he dislikes. Unlike Ren, he is shown to be highly competent and an evil genius.
At first, he was just shown to constantly tease others, but after Stimpy comes back from a walk with Indifferent Ren, his overly negative traits come out. He is shown to not care much for his clone, spitting in his face. He also didn't show much care for Stimpy, putting many of Stimpy's prized possessions into a vat of acid before tying him and Indifferent Ren to a metal board. He is shown a desire of world domination and tried to replicate himself in order to do so. However, by doing so, he ended up creating the female Hideously Evil Ren, who he fell in love with and ended up marrying.
As he didn't have an indifferent side, he had no self control and did horrendous acts despite Stimpy's begging. He was also shown to be rude and disrespectful, as when a Girl Scout arrived and asked him if he wanted her cookies, he chewed them up, spit them back into the bag, and refused.
Unlike the regular Ren, he is shown to share Stimpy's knowledge of science and chemistry, able to create a vat of acid within twelve hours of Stimpy and Indifferent Ren's absence. He manages to recreate the replicating chemical, XB-49 from scratch. Evil Ren was also able to convert the house into his evil lair within this timeframe, which included many staircases, a metal board, and the aforementioned vat of acid. He is also shown to be incredibly evil and sadistic, getting glee from seeing Stimpy or Indifferent Ren in pain, while constantly laughing evilly. He lured the pair into thinking that he should be put back together, only to explain that he loved being evil, and splashed the chemical onto him, madly laughing.
However, he is shown to have a soft spot for Hideously Evil Ren, as the first time he saw her, he didn't attack her and instantly fell in love, calling her a 'tomato'.
Evil Ren makes his only appearance in "Ren's Bitter Half." In the episode, Ren comes into Stimpy's lab, demanding dinner and grabs Stimpy's replicating chemical, XB-49. He begins to shake it and some of it spills onto him, splitting him into his two opposite halves: Evil Ren and Indifferent Ren. Stimpy, while afraid, is overjoyed, as he gets two best friends.
Later, the trio watch television, with Evil Ren continuously punching Indifferent Ren. After Stimpy asks if he is tired of watching cartoons, he brings the television closer by throwing it onto them and plugs it in, laughing evilly.
Later, Stimpy decides to take Indifferent Ren for a walk and asks him if he wants to accompany them, to which he silently refuses. After they leave, he boards up the door with planks. Twelve hours later, they return to find that Evil Ren has completely succumbed to madness, turning the house into an evil lair, dressing in clothes and building a vat of acid. He begins to throw Stimpy's possessions into the vat, including his favorite record, his catnip mouse, Mousey, and, after teasing him, his litter box. Soon, Stimpy faints, so Evil Ren takes the opportunity to strap him and Indifferent Ren to a metal board, where Stimpy sees him tinkering with chemicals and making XB-49. He attempts to reason with him, saying that he and Indifferent Ren need to be back together, otherwise they are each half a man. He promises that he'll help them if they allow him, and for a second, Evil Ren genuinely seems regretful, but soon reveals that he hasn't changed and that he likes being evil and spills the chemical onto himself, laughing maniacally. Soon, his first clone, Hideously Evil Ren, is created, and upon seeing her, falls in love.
Some time later, the pair marry, and proceed to slap each other after the ceremony. Hideously Evil Ren then puts them into their car and they proceed to drive off, with Indifferent Ren and Stimpy tied to the back.
He looks exactly like Ren, almost always not wearing clothes. However, as the episode progresses, he is shown to dress in a white shirt with white pants, a black vest with a golden belt and buttons, black boots and gloves, and occasionally wearing a black cap and goggles. He also wears a red armband with an 'R' symbol on it. He is almost always shown to have a devilish grin. For his wedding, he wears a black suit with black coattails, a red bowtie and a black hat.
- According to Stimpy, without the influence of Indifferent Ren, Evil Ren would become more villainous and corrupted, which is shown by him desiring to create an army of clones to conquer the world and displaying dictator-like traits.
- This makes it possible that Ren being indifferent keeps him from being worse than he already is.
- He is shown to display several Nazi-like traits, making it possible that Nazism was an inspiration for Evil Ren. In the episode, he displays flags with 'R' on them that have resemblance to the Nazi symbol and in Stimpy's visions, leads a Nazi-like walk before taking over the world, marking it in his symbol. Indeed, one goal of Germany during the Nazi-era was to expand their empire. He also is shown to wear a red armband with his symbol, which was common for the Nazis to wear a red armband over their clothes with the Nazi symbol. In one scene, he also puts his left arm up in a gesture similar to the Hitler salute.
- As noted by some viewers, Evil Ren also portrays characteristics similar to the stereotypes of communists. At the time "Ren's Bitter Half" aired, the United States was still heavily involved in conflicts with Russia.
- Unlike his clone, he is shown to look exactly like Ren Hoek, the only difference being that Evil Ren wears clothes more often.