Character.AI is built on a proprietary large language model trained specifically for character consistency, persona adherence, and long-form conversational coherence. The platform uses a character definition system (name, greeting, description, and example dialogues) to condition the model's outputs at inference time. Characters are served via a cloud-hosted web app and native mobile clients (iOS/Android); there is no publicly documented REST API for third-party integrations as of mid-2024. The backend leverages transformer-based architectures fine-tuned with reinforcement learning from human feedback (RLHF) to maintain persona fidelity across multi-turn conversations.