The AI Tau is a fragment of the Alpha, the AI given to Project Freelancer for use. She was the representation of the Alphas pain. She often let out bursts of painful electricity due to certain complications with her programming.
Initially she was given to Agent Utah, who was in possession of the Overshields armor enhancement. She was aware of the relationship between Agent California and her owner, which intrigued her. Following the discharge of Agent Utah, the ex-agent passed on her AI and armor enhancement to Agent California, with the Director's authority.
This proved to be beneficial for Cal, as she helped him through his pain while he simoultaneously helped with hers. The partnership also benefitted his skills, increasing his overall speed and the strength of his overshields.
It is confirmed that Cal managed to hold onto Tau for a majority of his life before passing it onto his grandson, the new Agent Arkansas.