Reaper & Reapette
Reapers are among the more powerful minions of the Underworld Army, and are responsible for ferrying the souls of the dead to the underworld. They first appeared in the original Kid Icarus with a fairly generic "grim reaper" design. They patrol their assigned areas slowly, but upon seeing Pit, they will go berserk and summon a swarm of flying Reapettes to attack, accompanied by frantic music.
In Kid Icarus Uprising, Reapers return with a more distinctive design, though their behavior remains similar. They are prominently featured in the fourth chapter, where Pit must infiltrate their fortress. The boss of the area is the gigantic Great Reaper.
A Reaper appears in Brawl as a sticker.
In Smash Run
Reapers appear as enemies, using their design from Kid Icarus: Uprising. They remain stationary and look around themselves, and upon noticing a player, they start attacking, sending out homing Reapettes which make characters trip on contact. If close enough, they also attack with a scythe swipe which does high damage and knockback. While a Reaper is activated, a whimsical 8-bit tune (the same heard in the Kid Icarus series in the same circumstance) overrides any prior music.
The Reaper trophy is a standard random unlock in the 3DS version.
In Smash Tour
A Reaper can appear as a random event. It roams around the board, and when touched by a player, it grants them a checkpoint bonus in exchange for three fighters before disappearing. Players with fewer than three characters just get launched away.
Reaper & Reapette appears as a Spirit for the game's Spirit mode.