The Reaper is the very first Fixed-Wing to be unlocked. Their behavior is different from Thunderbolts and Raptors as they fly around their target until it is destroyed, rather than attacking it with a strike before leaving. This is also shared by the MQ-11 Avenger.
Flak Gun and SAM are able to easily destroy them, so one should keep them occupied with high-health aircraft (or heavy vehicles, if they target ground units), and target critical defensive structures that cannot reach them, such as the railgun.
The Reapers deal low damage individually, but this can be remedied with equipment and upgrades. Even then, they can pose a threat to units with lower health such as helicopters or even chip down tanks (that can't attack it), though they do not fare well against Infantry, as a group of Riflemen can easily take them out.
If deployed as a defensive unit from the airfield, the Reapers will arrive from the edge of the screen, and after that, will circle around it (even if the building was destroyed) and attack anything that gets too close to them. Each time it spots a target, it will launch a missile at it and occasionally draw fire from enemy units that can return fire