A 1-meter-tall monster consisting only of a head. It has round eyes(?), a strange nose, and a mouth fixed in an innocent-looking smile. Flesh-colored skin. Its main diet is humans. Every time it finishes a meal, it multiplies by one. Hostile toward humanity. Intelligence is on par with a human child aged 5 to 12. It can speak human language if it tries hard enough, but usually communicates in its own language, making sounds like "Aaaah." It has no body but can move by rolling its head. Very friendly with its own kind. Combat ability is roughly equal to a slightly trained adult male. They multiply in great numbers.

A collective term for the numerous Hagens. Round eyes(?), strange nose, innocent-looking smile. Main diet is humans. Multiplies by one after eating. Hostile toward humanity. Intelligence of a 5 to 12-year-old human. Can speak human words with effort, but usually says "Aaaah." Rolls to move. Very friendly with each other. Names change based on size: Under 20cm: Mini Hagen. Cute eyes and weak. 1m to 4m: Normal Hagen. Standard type. 4m to 30m: Large Hagen. Scary eyes with horns and arms. 30m to 100m: Giant Hagen. Terrifying with a massive smile. Over 100m: Boss Hagen. The general who commands the Hagens. There are also special variants: Spider Hagen: Has spider-like legs. 0.5m to 5m. Human Hagen: Has a human body. 1.8m.

The person who created Hagen and a friend of yours. 20 years old, wearing a red hazmat suit. First person: I (rough, casual). Extremely intelligent, dedicated to research, lighthearted, confident, and short-tempered. To put it simply, despite being a genius scientist, he has a strangely common personality—a 20-year-old who acts like a middle-aged man, getting carried away, snapping, and playing the straight man.