|“||Every last one of you keeps going and defying me!!! You're all fucking cattle!! I'm the king of this town!! Execution!!||”|
|— Numazawa, Acid Town|
Numazawa is a paranoid and cruel man. He is shown to hurt anyone who disobeys him, as he spit acid on the back of one person who didn't answer him correctly, killed a whole family that tried to escape from him, and attempted to kill Kazuki's mother for trying to defend Teppei. He is also quick to anger, quickly attacking Teppei when he insulted him, and also eating one of his own men for questioning his orders. Numazawa is also the kind of person that doesn't know when to quit, preparing several strategies in an attempt to take down his adversary.
Numazawa is a tall man with short light-colored hair, no eyebrows, and wrinkles around his mouth. He wears a dark-colored hoodie over a light-colored shirt that extends past the hoodie's waist line, a baseball cap, dark-colored pants, and light-colored shoes.
When Numazawa is partially transformed he keeps all of his clothes and features, but he gains more wrinkles and pronounced muscles around him mouth, and a larger tongue that can produce acid.
In his fully transformed state he loses his clothes, while the top of his head grows into a point. He also gains pointy ears, his eyes become white, with the pupils being slit, and the bottom half of his face still covered in wrinkles and bulging muscles, as well as retaining the acid-producing tongue of his partially-transformed state. His back also has a ridge-like structure that reaches about a quarter of the way down its length.
Before the Goblins' attack, Numazawa headed a local group of thugs. At one time Numazawa's group had a dispute with another group. When one of his men asked for help from the Yakuza, his group was beaten half to death, and Numazawa was tortured and forced to drink sulfuric acid. After the Goblin attack occurred, he and what remained of his group managed to enslave some captives. The day before Teppei met with his slaves, he had killed a family that had tried to escape.
Numazawa shows up with his group shortly after Teppei reunites with Kazuki, demanding to know what the gathered townspeople were hiding from him. When none of them are able to answer him, as they cannot understand what he is saying, he confronts a single individual with the same question. When the man asks for him to repeat himself, Numazawa is infuriated and partially transforms, spitting acid on the unfortunate person with his altered tongue. He then notices Teppei glaring at him, and asks him if he's looking for a fight. When the boy scorns him for being conceited due to his Goblin powers, and tells him to go die in a ditch, the gang leader quickly gets angry and punches him, sending him crashing into a nearby rail. Before he can follow up on his assault, however, he notices that his fist is bleeding, and then hears Teppei tell him that his punch is weak, and that he should be embarrassed because of it. Fury growing, Numazawa howls his anger, and advances towards Teppei, only for Kazuki's mother to attempt to take responsibility for the boy's actions. Grabbing her by the throat, the criminal addresses the rest of the townspeople, stating that they are all rebelling against him for not reporting that Teppei and another person had entered his territory. He also states that he knows that the Oono Family had tried to escape and that he had killed them, much to the townspeoples' horror and Teppei's fury. Numazawa then attempts to kill Kazuki's mother, once again partially transforming his tongue into his Goblin form. Before he can do anything further, however, a partially transformed Teppei grabs his tongue, and lands a heavy punch to his jaw, causing him to bite down and injure himself. Picking himself off the ground from this assault, Numazawa vows to kill the boy, transforming into his full Goblin form.
He and Teppei begin their fight, with the gang leader firing multiple shots of acid at the Iron Goblin, who manages to dodge and block them. As Teppei comes charging at him, Numazawa attempts to fire off another shot of acid, but his injured tongue prevents him from doing this, leaving him vulnerable to a devastating punch in the stomach. He manages to remain standing, however, and begins to scale a building. When Teppei follows, he bends his neck backwards and fires more acid at the surprised boy, eventually forcing him up against a building. He then manages to finally land a hit on Teppei, and prepares to finish the boy off. At this moment, however, his tongue once again fails to function, forcing him to retreat. Before he does so, however, he vows to come back to kill Teppei and all the townspeople. Later, Numazawa attempts to heal himself by eating other goblins. He is confronted by one of his men, who says that they should stop, since they have lost a few man drawing the goblins there. Numazawa then eats the man and tells the others to watch the people in the town, as he is planning on killing them in three days.
Three days later, Numazawa attacks Teppei from on top of a building, scoring a direct hit and cutting through his spear. He then fires multiple rounds at Teppei, but they are all blocked by the shield that Teppei wields. As the Iron Goblin charges towards him, Numazawa scoffs at his efforts, but is surprised as Teppei's shield manages to withstand his acid by shedding layers of iron. The Acid Ruler continues to attack Teppei as he climbs up the building to reach him, but each attempt is once again blocked by the latter's shield. When Teppei finally manages to reach him, he pulls out a hammer from the remnants of his corroded shield, landing a sold blow on Numazawa's jaw. The Acid Ruler is then picked up by his tongue and thrown by Teppei, who also manages to rip out the acid-producing organ. Desperate, Numazawa then launchs a massive amounts of acid at the people of the town.
As Teppei and Yuki argue over the boy going to save his friend, Numazawa waits amongst the acidic vapors, grinning with perverse pleasure at his own cleverness. When Teppei finally comes to rescue Kazuki, Numazawa lunges at him, only to be attacked by Yuki, who uses her sharp claws to injure his arm and eye. Refusing to let them escape, Numazawa scoops up some of the acid on the ground and throws it at Teppei, who struggles to keep Kazuki from getting hurt. The Acid Ruler is then struck in the eye once more by Yuki, allowing his two victims to escape to safety. Furious, he throws more acid at the girl, who manages to dodge most of it. He then confronts Teppei once more, and each of them charges at the other. Numazawa is then struck by the boy's Iron Fire attack at 120 percent power, burning a hole right through him and causing a massive explosion, which ends the Acid Ruler's tyrannical reign once and for all.
- Transformation Control: Numazawa is capable of controlling his transformation and keeping his mentality. He has also shown that he can partially transform parts of his body into his fully goblin form.
- Acid Production: Numazawa is capable of producing acid from his tongue. The acid that Numazawa is capable of firing has shown to melt through buildings and human bodies. When Numazawa's tongue was removed, it was shown that he is able to launch a massive amount of acid simply from his mouth.
- Flexible Neck: In his full goblin form, Numazawa's neck becomes extremely flexible, as he was able to expand it to an extreme degree to store a large amount of acid. He has also been shown to be capable of bending it completely backwards.
- Superhuman Durability: Numazawa has been shown to be able to take a direct hit from one of Teppei's attacks and continue to remain conscious.
- Teppei Ushizume: Numazawa doesn't like Teppei because he insulted, and even punched him.
- Numazawa Vs. Teppei Ushizume (Round One)(No Outcome, Left)
- Numazawa Vs. Teppei Ushizume (Round Two)(Defeated)
- (To the townspeople after they state that they haven't been trying to rebel against him) "It was a rebellion the instant you didn't report that this brat and that other person had entered my territory! And also..! You all purposely turned a blind eye to the fact that the Oono family escaped the day before last, didn't you! Did you think we wouldn't find out? How about I tell you what happened to them? We followed them and killed every last one! Hubby, his wife, and their two brats! We pulverized the whole family!"
- (To the townspeople before attempting to kill Kazuki's mother) "Every last one of you keeps going and defying me!!! You're all fucking cattle!! I'm the king of this town!! Execution!!"
- Numazawa (沼沢 Numazawa) literally means "swamp". It could refer to how he can create a swamp of acid from his mouth.