Maid is Ryuunosuke's artificial intelligence (AI) program he wrote personally. Originally, Ryuunosuke designed Maid to filter and automatically reply to his cellphone messages and emails to save him time. However, Ryuunosuke keeps expanding Maid's AI ability. Maid is now capable of blocking and replying to a message or email by her own preference, using new words she has recently learned from other messages, emails, or the Internet in the reply, giving Sorata solid advice for his computer game program, anticipating people's emotions with cameras, writing computer or cellphone viruses and sending them out by herself, or even hacking into Suiko's computer to steal information. The only way to distinguish Maid's message from those from a human is that she replies too quickly. Ryuunosuke lets Maid enter a social networking service, with most people who meet her not realizing she is an AI program. According to Ryuunosuke, his ultimate goal is to give Maid an artificial body and become a real human.

(Source: Sakurasou no Pet na Kanojo Wikia)