Mokujin has no gender, so I will use the neutral feminine to refer to her. She is a character in Tekken 3, and is what her name implies: "wooden person".

When you choose to play Mokujin, the game randomly selects one of the other characters for her to emulate. This choice being RANDOM, you have no control over it. Usually, you can identify which character she emulates by her stance, etc. She does not have a fixed shape, but rather it changes slightly, depending on who she is emulating. If she appears to be holding a sword, that is a dead giveaway that she is emulating Yoshimitsu.