Guard is a minor character in Phantom Dust. He is playable by highlighting Meister and holding both triggers then pressing A. He can evade attacks by tumbling. When you don't move around or use any skills for a little while, he starts to dance that is made fun of online a lot.
Guard is a robot with advanced yet, basic, AI and he is discovered wandering around on the surface by Meister. He is a friendly robot that knows how to use the dust and he mimics the attacks that he sees used by other characters. Know is given the job of training him and takes the protagonist on many trips to train Guard. When you select him as a partner in the single player missions, Guard will bring an exact copy of your arsenal. After you beat the game, in the simulator, he uses his own [very peculiar] arsenal which consists of mainly Ki and Psycho skills.
Prepare yourself, you'll be fighting Guard often as he tends to be a little rebellious and goes through a bit of stress (robots feel stress?). A couple of times Guard runs away from Know and the underground city. This is in part to Guard going a bit haywire. The cause of this outburst is not explained in game and since Guard can hardly speak intelligible words (!!!$!^@&!*##~!) it is very difficult to gather what is wrong with him.
After reading some of the memory boxes found in the game, there is a theory that Guard is some odd form of Edgar's spaceship computer.