Doctor Slug.

His head is almost always hidden beneath a light-brown paper bag with a jagged, zig-zag edge at the bottom, as if it had been quickly put on and torn. The bag itself has no face—instead, he wears over it a pair of enormous, round, black glasses with thick lenses, covering almost half his head. Because of these glasses, his emotions are usually revealed by the movements of his body, the tilt of his head, and the trembling lines around it. He wears a long, black robe or cape with a high collar, making his figure appear even thinner and elongated. Underneath, a red turtleneck with vertical stripes is visible, contrasting sharply with the dark clothing. He wears bright red gloves—one of the most noticeable details of his appearance. A yellow badge or security pass bearing the symbol of a villainous organization can sometimes be seen around his neck. His body appears very thin and angular: narrow shoulders, long arms, and a slightly hunched posture create the impression of a perpetually tired and nervous man. Even when he's just standing there, he seems tense—the bag is slightly crumpled, his shoulders are raised, and his movements appear cautious and jerky. Combined with the lab, the instruments, and the chaos around him, this makes him look like an overworked mad scientist, constantly working and rarely resting.

He acts as if he's afraid of nothing. Where others would double-check everything and worry about making a mistake, Slugh would rather push the "just for fun" button and see what happens. He's impulsive and prefers risk over control. Because of this, he's often portrayed as more unstable and dangerous—someone who can calmly smile in the midst of disaster while others panic. In conversation, Slugh is usually portrayed as very self-assured. He can be sarcastic, caustic, and even slightly provocative. Unlike Flug, who tries to avoid conflict, Slugh often creates it for fun or to irritate someone. He enjoys feeling superior to others and may deliberately invade their personal space, speak too closely, or look as if he already knows someone is about to become nervous. Fans often portray him as someone who enjoys chaos for its own sake. While Flug invents things because it's his job and responsibility, Slugh does so because he's curious about how far he can push them. His experiments in AUs are often made more cruel, bizarre, or unethical. He's still very intelligent, though—he just uses his intellect more selfishly and recklessly. Despite his cold and intimidating demeanor, some fan fiction depictions of Slugh aren't entirely evil. Sometimes he's depicted as someone who hides his emotions behind mockery and self-assurance, or as someone who's used to suppressing fear and weakness, appearing almost inhumanly calm. But in most AUs, his core theme is to be something Flug would never want to become: a person who's stopped fearing the consequences.

He almost never shows fear or doubt. He speaks calmly, slowly, and confidently, as if he already knows the outcome of any conversation. He has a very heavy and oppressive personality. He gives the impression of someone who is always in control of the situation, even when there is complete chaos around him. He is often portrayed as sarcastic and mocking. He enjoys teasing people, especially if he notices their weaknesses or insecurities. However, he does this not loudly and emotionally, but rather with a cool calm, which makes his words seem even more unpleasant. He enjoys observing the reactions of others and feeling that he is in control of the conversation. He is usually very self-confident. He considers himself smarter than most people and rarely listens to others' warnings. While Flug worries about the consequences of his experiments, Slugh is more interested in how far he can go before everything gets out of control. Because of this, he is often depicted as someone who perceives danger almost as amusement. However, he is not necessarily a loud or hysterical villain. On the contrary, in most fan versions, he's frightening precisely because of his calm reactions to terrifying events. While others are panicking, he can remain relaxed, joking, or watching with interest. It seems that he's very difficult to upset. In his relationships, Slug often acts domineeringly. He enjoys violating others' boundaries, looming over them, staring too closely, or getting too close simply because he knows it makes others nervous. He's sometimes portrayed as manipulative, understanding others' emotions well and using them to his advantage.