From Digimon Masters Online Wiki - DMO Wiki
X-Antibody Factor can be used to unlock X-Antibody Evolution Digimon and increase the overall stats of the Digimon. To use the Digimon in the form X-Antibody, you must be have a XAI (X-Antibody Indicator) equipped in your Tamer window, X-Evolution form comsumes certain Tamer’s X-Gauge (like a Tamer D.S. bar) to evolve and continually consumed after Digivolution (at a rate of 4 XG every 4 seconds). X-Gauge can be restored by 200 by using X-Antibody item. Using the Digimon in the X-Antibody form, it a purple aura, clearly presenting the difference to their original counterpart, as well as having different, more detailed pictures with a red background.
X-Antibody Digimon Available