The Mercy evolved from early maintenance shuttles used during the construction of the Mothership. Eventually the old shuttle design would serve as the new Mercy with a few modifications including a small docking bay on its underside as well as a projectile cannon turret.
The Mercy, used to repair strike craft and capital ships alike, would serve combat duty throughout the entire Homeworld War. Newly equipped with a reversed version of the phased disassembler array, the Mercy literally breaks down damaged material and restructures it. As such it can repair enemy ships low of cost. This takes a very long time but is worth the effort and can be used effectively to heal other corvettes as well as capital ships while fighters line up to dock with the Mercy.