The Harpy is one of two pets currently in the game utilizing ranged attacks based on P.atk, and not M.atk (the other being the Joker). The Harpy boasts a better attack range than every other pet in the game; a range slightly superior to the Shadow Hunter character, and only slightly less than that of the Archer.
Harpies are best designed to dispatch enemies at a distance, stunning or slowing movement speed of mobs with a debuff. They are best equipped with a longbow or crossbow with their ranged weapon passive attained at level 90.
The Harpy also makes an excellent dual-summon pet in dungeon parties, particularly with Evoker (utilizing dual-axes), or Battle Summoner classes. A Battle Summoner's Might of the Underworld skill will decrease the Harpy's Vitality and Strength, yet boosts those same stats of the summoner; a tradeoff which scarcely affects the Harpy, as pets can be Stabilized, reducing the amount of aggro generated. Also, with a Harpy's Damage Dealing mainly contributed to by Dexterity, losing Strength is a not an overly a concern, as the boost to the Battle Summoner's damage output is considerable.
Harpy Unity, a boost to physical attack speed, also makes the Harpy Potion (available at Evo.3) a popular choice for classes (such as Assassin) who rely on attack speed. Overall, the Harpy pet is popular across a range of character classes, their ranged attacks and versatility making them an excellent choice for Shadow Hunters and Archers.