The story revolves around Kenzo Tenma, a Japanese surgeon living in Germany whose life enters turmoil after getting himself involved with Johan Liebert, one of his former patients who is revealed to be a dangerous psychopath.
When a young boy with a bullet wound arrives in 1986, Tenma is about to operate when Chief of Surgery Dr. Oppenheim and Director Heinemann tell him to work on the mayor, who came in later instead.
Plagued by guilt, he resolves to find Johan and end the life of this "monster" he feels responsible for creating, while evading Inspector Lunge, who suspects Tenma of the murders.
He possesses charisma and intelligence but is also cunning, manipulative and deceitful; while he is seemingly kind, compassionate and loving to children, he is actually cold, cruel and fundamentally incapable of empathy.
Johan placed first in Paste's list of the "20 of the Greatest Villains in Anime", with Toussaint Egan saying that him the "Despite all that he was able to accomplish over the course of Naoki Urasawa’s Monster — manipulating over fifty people to kill one another through suggestion alone, framing the man who saved his life for murder, operating a massive money laundering operation at the heart of Germany, and coordinating a shadow network of killers to do his bidding— Johan has no special abilities to speak of, save for intelligence.
Voiced by: Tsutomu Isobe (Japanese); Richard Epcar (English) Heinrich Lunge (ハインリッヒ・ルンゲ, Hainrihhi Runge)[3] is a BKA detective assigned to the murder case at Eisler Memorial Hospital.
Voiced by: Junko Takeuchi (Japanese); Laura Bailey (English) Dieter (ディーター, Dītā) is a young boy Tenma encounters in his search for Johan.
In Another Monster, Eva explains that she left Tenma after Director Heinemann demoted him because her father needed a right-hand man who he could put his full trust in.
Voiced by: Nobuaki Fukuda (Japanese); Kirk Thornton (English) Dr. Oppenheim (オッペンハイム, Oppenhaimu) is the Chief of Surgery at Eisler Memorial, serving under Udo Heinemann.
When Anna Liebert collapses near Johan, Boyer is instructed to photograph her memory jog over Tenma's objections as part of Director Heinemann's "emergency image-saver" for retrieving hospital's costs.
While Dr. Tenma is treating opera singer F. Rosenbach on Dr. Oppenheim's orders, Dr. Becker handles the surgery of a Turkish construction worker who dies from treatment delays.
Voiced by: Ryuji Nakagi (Japanese); David Lodge (English) Benjamin Weisbach (ベンヤミン・ヴァイスバッハ, Ben'yamin Vaisubahha) is a Nordrhein-Westfalen police detective who is the first person to investigate the Lieberts' murder.
Voiced by: Takayuki Sugo (Japanese); Derek Stephen Prince (English) Rudi Gillen (ルディー・ギレン, Rudī Giren) is a criminologist and one of Tenma's former classmates at university (a former student of Dr. Reichwein).
Voiced by: Nobuyuki Katsube (Japanese); JB Blanc (English) Roberto (ロベルト, Roberuto) is a big, burly man and one of the many people from the 511 Kinderheim orphanage whom Johan controls.
Lanke tells a story about his sister who, with her husband, was shot trying to cross the Berlin Wall into West Germany; only their son, Adolf Reinhart (アドルフ・ラインハルト, Adorufu Rainharuto), survived the attack.
Voiced by: Michio Hazama (Japanese); Dan Woren (English) Hans Georg Schuwald (ハンス・ゲオルグ・シューバルト, Hansu Georugu Shūbaruto), known as the "Vampire of Bavaria" (バイエルンの吸血鬼, Baierun no Kyūketsuki), is a successful, reclusive paraplegic businessman who has strong influence on the European economy.
Voiced by: Tomokazu Seki (Japanese); Yuri Lowenthal (English) Karl Neumann (カール・ノイマン, Kāru Noiman) is a student at the University of Munich and the son of Schuwald and Langer (who sent him away for a better life).
Voiced by: Kyoko Hikami (Japanese); Julie Ann Taylor (English) Lotte Frank (ロッテ・フランク, Rotte Furanku) is a student at the University of Munich who is trying to win Karl Neumann's heart, so she aids his investigation of his father and late mother.
Voiced by: Ichirō Nagai (Japanese); Paul St. Peter (English) Julius Reichwein (ユーリウス・ライヒワイン, Yūriusu Raihiwain) is a psychologist who specializes in counseling and assisting recovering alcoholics.
Voiced by: Hideyuki Tanaka (Japanese); Patrick Seitz (English) Wolfgang Grimmer (ヴォルフガング・グリマー, Vorufugangu Gurimā) is a freelance journalist researching 511 Kinderheim, who is drawn into the search for Johan when he helps Tenma.
However he seems to have a dark side; a former subject in 511 Kinderheim, he has developed another personality: an aggressive fighter who protects him when he is under stress, whom he calls "Magnificent Steiner" (超人シュタイナー), after a television show he watched as a child.
Voiced by: Yoshisada Sakaguchi (Japanese); Peter Lurie (English) Karel Lanke (カレル・ランケ, Kareru Ranke) is a former captain in the Czechoslovak secret police who wants to see Tenma and Grimmer.
Voiced by: Ryūsuke Ōbayashi (Japanese); Kyle Hebert (English) Fritz Verdemann (フリッツ・バーデマン, Furittsu Bādeman) is a lawyer hired by residents of Düsseldorf after Tenma is arrested in Prague.
Voiced by: Hiroaki Hirata (Japanese); Troy Baker (English) Jaromir Lipski (ジャロミール・リプスキー, Jaromīru Ripusukī) is a down-and-out puppeteer in Prague, who meets Nina as she passes through the city in pursuit of Johan.
Voiced by: Shuuichi Ikeda (Japanese); Roger Craig Smith (English) Martin Reest (マルティン・レースト, Marutin Rēsuto) was imprisoned for eight years for shooting his girlfriend and her lover.
Voiced by: Nobuo Tanaka (Japanese); Dave Mallow (English) Peter Čapek (ペトル・チャペック, Petoru Chapekku) is the last individual in the organization: a bespectacled, white-haired, mysterious man responsible for many incidents in Monster.
Voiced by: Nachi Nozawa (Japanese); Michael McConnohie (English) Franz Bonaparta (フランツ・ボナパルタ, Furantsu Bonaparuta) is the man responsible for the eugenics experiment leading to the birth of the Liebert twins, and author of the storybooks used to indoctrinate the children of the Red Rose Mansion; notably The Nameless Monster, from which Johan took his name and his mode of operation.
116, 149] Voiced by: Akiko Yajima (Japanese); Colleen O'Shaughnessey (English) Wim Gnaup (ヴィム・クナウプ) lives in Ruhenheim and works at a hotel owned by Franz Bonaparta.
Voiced by: Eiji Hanawa (Japanese); Doug Erholtz (English) Herbert Gnaup (ヘルベルト・クナウプ) is an unemployed alcoholic who is the town drunk and the neglectful father of Wim.
Voiced by: Gara Takashima, Houko Kuwashima (young) (Japanese); Lauren Landa (English) Věra Černá is the mother of Johan and Anna/Nina, and a friend of Margot Langer.