The Crooked Man is the monster who frequently tries to kill David during the course of the game.
His real name is Duke MacGahan (デゥーク・マクガーン Dukku Makugaan) and he is the tenant David's been looking for in the whole game. He commited suicide years ago and he is the true identity of the Crooked Man.
If his facets and the notes he wrote during the course of the game are to be believed, Duke was an idealist young man, who due to his troubled past (being abandoned by his mother, failing to become a laywer and being dumped by his girlfriend) became depressed and eventually commited suicide.
After becoming a vengeful spirit, he becomes resentful and upon seeing David, he immediately dislikes him. The reason he follows David is because he hates him for being so much like him and wants him to die for it. And because some part of him wants David to not go down the same path he did and end up like him.
As the Crooked Man, he generally goes after David, but if Sissi, D or Fluffy are around he prefers going after them and trying to kill them much to David's confusion. After it is revealed that the three of them are facets of Duke, it makes perfect sense. Sissi has regrets over lost love, D has regrets over failed dreams, and Fluffy's been abandoned. These are all the aspects of Duke that ultimately led to his eventual suicide.
Sissi, D and Fluffy are facets of his own past life, and so their background told during the course of the story was actually his.
When he was young, he was "abandoned" by his mother. The facts about it are ambiguous, but apparently she died in a hospital when Duke was a kid, leaving him with bitterness and feelings of abandonment. Years later, he decided to become a lawyer, but no matter how much he tried, he never passed the final exam. The last straw for him was that his beloved girlfriend, whom he loved very much to the point of making her gifts and taking her to their "special place" in a hotel; eventually abandoned him. This left him very depressed.
Unable to take it any longer, he left the apartment he lived in, went to his mother’s abandoned house, where he committed suicide by hanging himself. He becomes a vengeful spirit later.
After David moves to the apartment Duke used to live in, he frequently haunts him. David deduces that the house’s malfunctions have something to do with the former tenant and decides to look for him. Duke, as the Crooked Man, starts following David everywhere on his journey to find Duke, and everytime tries to kill him. He is also very resistant to David’s attacks, as during the game he gets shoved into a pit, set on fire, stabbed, locked into a room and shot and still follows David around.
In the Bad End 1 "What a sissy", if David tells Sissi not to cry over her lost love, she decides to give up on her quest to keep fond memories alive and drops the precious box her boyfriend gave her. Because of the time Sissi took to talk, the Crooked Man had enough time to revive. As she is about to leave, she is attacked and brutally killed off-screen by the Crooked Man, who then kills David.
In the Bad End 2 "Never, never give up", if David tells D not to give up on his failed dreams to become a lawyer, D is angered, claims David cannot understand him and stabs him to death. He is later killed by the Crooked Man. The Crooked Man then slowly walks over to David, speaks briefly, then walks away as the screen fades.
After David saves Fluffy in the hospital, the Crooked Man takes over him and forces him to take the gun and commit suicide. Meanwhile, Sissi, Fluffy and D help Paul in finding David, who is broken, crying and about to shoot himself. In the Bad End 3 "Goodbye, my friend", if Paul tries to persuade him, David says that he doesn’t understand his pain and shoots himself, much to Paul’s despair. In the Bad End 4 "It’s not your fault, David", if Paul decides to take the gun from David, the two struggle and David accidentally shoots Paul to death. David later takes his own life. If Paul decides to punch David, Paul punches David repeatitly to get The Crooked Man out of David's body. It then works and David tells his story.
After Paul saves David, he goes to the abandoned house of the former tenant, which actually belonged to Duke's mother, who abandoned him. When looking in the belongings, he finds out the tenant’s real name. Then The Crooked Man crys and realizes that David is watching him and screams. After shooting him enough times, David tells him to disappear, since he won’t take the same path as him. With these words, the creature disappears. David later enters the attic of the house, where he finds Duke's hanged corpse. David realizes that all the situations Duke put him in were in order to help him move on with his life and he thanks him before leaving.
Duke's corpse is later retrieved and put to rest in a cemetary, pressumably by David. In the final scene of the game, David visits Duke's grave, puts some Marlboro next to the grave and he says that they would see each other again.
In the second replay, when David enters to the attic, he has a conversation with Duke's spirit (who is recalling the moment he is about to commit suicide) before Duke passes on. His spirit is later seen after David and Shirley are back into their relationship, so it is implied he will continue looking after David.
- The Crooked Man represents Duke's resentment and despair. However, during Stage 3 there is a note from an otherwise-unrelated man who killed himself in order to get away from...something. David also says that he's been hearing "voices" in his head, long before the Crooked Man started coming for him. The Crooked Man in the game is Duke's, but apparently he's had one of his own for some time and it's part of the reason Duke decided to step in. It's even possible that the creature who tries to make David shoot himself in Stage 4 was David's own Crooked Man, not Duke's.