Companion

Companion is a Hunting  skill in Diablo III.

In-game
Passively summons a raven companion which follows the Demon Hunter around and may periodically peck the enemies for Physical damage. Once every 30 seconds, the skill may be activated to empower the raven's next attack, increasing its damage to.

For a player having trouble choosing between the rune options, unless otherwise noted, the damage dealt is weapon damage as Physical. Thus, for all intents and purposes, all that changes with the rune choice are the empowerment and passive bonuses offered.

Raven, Bat, Ferrets companions are not attackable; in return, they do not block enemy pathing. Other companions have collision size and efficiently block enemy movement, especially when summoned in large numbers. Moreover, all of them are immortal (even though they have Life bars), so they really help with tanking.

Runes

 * Spider Companion: summons a spider instead of a raven. Its attacks will debuff enemies with a  reduction in movement speed for  seconds, and it hits multiple enemies in front of itself per hit. Upon being empowered, the spider will throw webs at all enemies within  yards, Slowing their movement by  for  seconds.
 * Bat Companion: summons a bat instead of a raven. The bat grants additional  Hatred per second. Empowering the bat instantly grants  Hatred.
 * Boar Companion: summons a boar instead of a raven. This boar increases the Demon Hunter's (and all other allies' within 100 yards) Life regeneration (scales with level, up to at level 70). In addition, the boar increases the Demon Hunter's and allies' resistances to all damage types by . Empowering the boar causes it to charge to the Demon Hunter's location, then taunt all enemies within  yards to attack the companion for  seconds.
 * Ferret Companion: two very quick Ferrets are summoned, instead of the raven. Passively, the ferrets will scurry around, picking up gold in the process, giving the Demon Hunter a  bonus to Gold Find, and offer a  movement speed bonus to the Demon Hunter (cannot bring the total speed above the usual +25% limit). Upon being empowered, the ferrets collect all gold, power globes and health globes within  yards.
 * Wolf Companion: summons a wolf instead of the raven. Upon empowerment, the wolf will howl, multiplicatively increasing all damage done by allies within  yards by . Howl lasts  seconds. Like the spider, the wolf hits multiple enemies at once in front of itself.

Non-rune enhancements

 * Zoey's Secret (Legendary Belt): for each active Companion, the Demon Hunter takes 8-9% less damage.
 * Embodiment of the Marauder Set (Set Bonus for 2 items): summons all companions at once. Empowering them will grant all six effects simultaneously.
 * Embodiment of the Marauder Set (Set Bonus for 6 items): for every active Sentry, damage of Companion increases by +12000%.
 * The Cloak of the Garwulf (Legendary Cloak): Wolf Companion summons three wolves. Does not give extra howls.
 * The Raven's Wing (Legendary Bow): summons one additional Raven, which does not attack, but picks up gold like ferrets.
 * Tasker and Theo (Legendary Gloves): increases Attack Speed of all Companions by 40-50%.
 * Enforcer (Legendary Gem): increases damage by 15% (+0.3% per rank) and reduces damage taken by 90% (rank 25 bonus).

Trivia

 * This skill is reminiscent of the World of Warcraft Hunter class mechanics, which allows one animal companion of player's choice to follow and assist the hero in battle.
 * Additionally, the Spider, Boar, and Wolf Companion idle animations use the idle animations of their counterpart beasts in World of Warcraft, which can be tamed by Hunters.
 * Originally there was a seventh, unused rune that summoned a Skull companion with an unknown effect.
 * A ferret companion was likely added as a tip of the hat towards the ferret pet from Torchlight I-II. Other schools of thought believe this to be a nod at the ferrets Kodo and Podo from the Beastmaster movie; those ferrets were known for thievery and were often referred to as the Beastmaster's "cunning".
 * Apparently, wolves were originally supposed to be monsters in game, as in-game files have multiple color schemes for them, way more than the skill uses. As of patch 2.4, Primeval Hunters appear on Greyhollow Island, and in Ruins of Sescheron, Aletur has a hunting pack of war-painted wolves with him.