The Mercurius is the second of the experimental mobile suits created for OZ by the five Gundam developers. Whereas the Vayeate was designed as an offensive mobile suit, the Mercurius was designed primarily as a defensive unit. As the defensive partner in this two-unit combination, the Mercurius is designed to protect the Vayeate from enemy attacks. It's test-piloted by Gundam pilot Heero Yuy, during his time as a prisoner of OZ. Months later, the resistance group White Fang uses Heero's recorded pilot data to create an unmanned mobile doll version of the Mercurius.

The Mercurius, built on the same basic frame as its Vayeate twin, was armed only with a small beam gun and a "crash shield," which was simply a large shield mounting a beam saber for close combat. The Mercurius' most important tool, however, was its group of "planet defensers," which were small hovering units whose positions and deployment could be remote-controlled by the Mercurius' pilot. These remote-controlled devices generate a protective field capable of deflecting both physical attacks and beam blasts. The planet defensers could be used to protect the Mercurius itself, or another unit , such as its offensive Vayeate brethren.

Source: Gundamproject.com, New Mobile Report Gundam Wing