The Reaper is a dedicated anti-air unit. It is built around the unique Arc Caster, possibly the most highly specialised of all AA weapons encountered by humanity. Its great accuracy is achieved as the weapon's beam automatically latches onto isolated targets. These same properties make it virtually useless against ground units, where the beam all too easily targets inanimate terrain rather than the intended victim.
The Reaper uses the Hunter's chassis and drive, and as such shares its blistering combat speed. This allows to actively hunt slower enemy fliers in a way that few other ground vehicles can match. This is just as well, since the Arc Caster has a very limited range.
Rules for this unit can be found in the Core Rulebook