Thornheart (ソーンハート) is the main antagonist of House of the Dead: Scarlet Dawn.
Thornheart's ancestors founded the DBR Corporation, and he was involved with Dr. Roy Curien and Caleb Goldman, the two other antagonists of the series. Thornheart was diagnosed with a rare disease, and by the events of the storyline his fate was unknown until Scarlet Dawn.
In an ending, Thornheart enters and sits down in a clean office not seen anywhere else in the game. He chastises Goldman for being "soft", claiming that humans "have no need of hope", and that the "true end" shall begin. He also mentions the existence of another "Pandora's Box".
In the "true" ending of the game, Thornheart limps in the chamber where the Magician was defeated. He states that Pandora's Box has been closed, if there is any hope left, and that the Wheel of Fate "cannot be stopped".
It is revealed that Thornheart was involved with Roy Curien and Caleb Goldman, and working on a mysterious project named "Noah's Ark". Thornheart's fate is unknown to the agents throughout most of the game, as they are unsure if he survived a deadly disease that he was diagnosed with years prior.
In the climax, they confront Thornheart, who reveals that he chose to be alive. He reveals that Noah's Ark is a plan devised by him to cleanse humanity, in which only the strongest survives. He unleashes his masterpiece, the Moon, in order to take down Kate and Ryan once and for all, and although the pair of them are unable to do any significant damage, Ryan stabs it with a rod and the creature is destroyed after being struck by a lightning bolt.
Thornheart appears in the two standard endings and first best ending, in which he is shown to have survived. In the first best ending, he reveals that the ordeal has just begun, before falling down and rising up, possibly becoming a zombie.
Thornheart appears in one ending, limping into the chamber where the Wheel of Fate was defeated. He picks up a glowing canister that was revealed earlier as Daniel's cure, chuckling to himself: "It appears that he didn't understand its true purpose." He then leaves.
- "It appears they didn't understand its true purpose."- from The House of the Dead III.
- "Goldman, you are soft. These wretched humans have no need for hope. Soon the true end shall begin. There is more than one Pandora's Box."- from The House of the Dead 4.
- "So Pandora's Box has been closed. That is if there is any "hope" left. The Wheel of Fate cannot be stopped"- from The House of the Dead 4: Special.
- "Curien, Goldman, it's time for humans to decide: Extinction or evolution?" From the intro to House of the Dead: Scarlet Dawn
- "There's more to come! Run until you die!" From the start of chapter 0 after unleashing the zombies on the guests from House of the Dead: Scarlet Dawn
- "It's impossible to change a predefined future. Your ordeal has only just begun." From House of the Dead: Scarlet Dawn
- Before his identity was revealed in Scarlet Dawn, House of the Dead fans often nicknamed Thornheart the "Mystery Man".
- His appearance varies throughout the games. In 4, he appears to be a middle-aged and normal-looking man, however in the sequel Scarlet Dawn (which takes place three years later), he is much older-looking, with pale skin, white hair and whitened eyes, which he looked like years before the game series began as evidenced in an old photograph. In 3 (which takes place 13 years after Scarlet Dawn) he appears to be slightly younger and back to normal-looking, although this was possibly due to the engine and the fact he is obscured and not to be fully seen. It is not yet explained why this is the case, although it is possible the condition he has in the storyline has something to do with it.
- His middle-aged, gaunt appearance, widow peak hairstyle, and suit resemble the character the G-Man from the Half-Life series. Both are mysterious entities whose identities are undisclosed.
- His appearance in Scarlet Dawn resembles actor Jon Voight.
- His laugh in III's ending is similar to Goldman's laugh from The House of the Dead 2.
- In a 2012 interview for The Website of the Dead, series director Takashi Oda stated that an arcade sequel would be based around the character.  Scarlet Dawn, released six years later, would reveal Thornheart's identity and establish him as the main antagonist.