Theodore Peterson, the titular Neighbor, is the primary antagonist. His AI is adaptive, meaning he learns from your actions. If you repeatedly enter through a specific window, he will eventually place a bear trap or board it up. If you always use the same route, he will patrol that area more frequently. Understanding this adaptive AI is the closest thing to a 'character build' for the antagonist.
The Neighbor has excellent hearing and vision. Loud noises (breaking glass, falling objects) will immediately alert him. He can also hear your footsteps if you're sprinting nearby. His vision cone is wide, and he can spot you from a distance, especially if you're not crouching. He will chase you relentlessly once spotted. If he catches you, you'll be returned to your house, and he will reset certain parts of the environment (e.g., re-locking doors, replacing traps). The key to dealing with the Neighbor is to constantly vary your approach, use distractions, and exploit his patrol patterns. His behavior also changes slightly between acts, becoming more aggressive and placing more complex traps in later stages.