Mercurius was an advanced gundanium-based mobile suit built for OZ, model number OZ-13MSX2. Its creators were the same scientists who built the five Gundams plaguing OZ, who had recently been captured.
Counterpart to Vayeate, the Mercurius suit was built to be highly defensive. To that end, it was equipped with a group of small devices that protected it (or another unit) with a powerful force field. The Mercurius was equipped with modest weaponry, compared to its brother. The Mercurius was tested by Heero Yuy, while he was also in the custody of OZ.
Following the escape of Heero and the other Gundam pilots, the Mercurius was refit as a mobile doll, using data recorded from Heero Yuy himself. It was destroyed while battling Duo Maxwell. Elements of both Mercurius and Vayeate were incorporated into the Virgo model.