Ennis, a decade before the series' time, was the lead alpha of his pack. He was recruited by the corrupt alpha, Deucalion to form the Alpha Pack. Ennis accepted the offer, killing all of his betas and his emissary to take their power to join, alongside fellow alpha Kali whom he was romantically involved with. In Season 3, Ennis, as part of the Alpha Pack, arrives in Beacon Hills having taken notice of Derek Hale's rise to Alpha status and Scott McCall's True Alpha potential. Ennis goes along with the alphas to assault, manipulate and dominate Derek's Pack for their own ends, kidnapping Derek's Pack and family Erica Reyes, Vernon Boyd and Cora Hale, offering Derek a place with them and igniting an all out battle with the Beacon Hills werewolves to this end. Ennis is injured in the scrap, then is betrayed by Deucalion who uses him to further his own plans for Derek and Scott and to take his own augented power, killing him.
Ennis is the strong, silent type, not very talkative, and prefers to take action instead of being diplomatic and reasonable. As the leader of his pack, he was arrogant, pompous and proud. An extremely aggressive werewolf, Ennis made amoral, barbaric decisions. He declared war with the Argents' hunter clan with the excuse that they killed one of his, eschewing responsibility that his beta made unwise decisions and had killed two of their people. He also decided, that to replace his fallen member, he'd recruit an innocent teenager, Paige Krasikeva, who was blind to the supernatural, viciously Biting her without her consent, with the insinuation he'd be in good graces with Talia Hale, because of Paige's tie to Derek Hale. Ennis' aggression and violent tendencies are proven further by his murdering his remaining pack to take their power, showing he could be corrupted and manipulated similarly to Kali, traits that obviously were the reason Deucalion recruited him. Ennis' pride and arrogance were only made worse by his place as part of the Alpha Pack.
Ten years, give or take, before the events of the series, Ennis was the lead alpha of his Pack. At some point, a naive and dangerous beta of his unwisely came to be in the company of two of the Argents' people, and he accidentally killed them.
"They dragged him here. An arrow in his throat. They hung him. Cut him in half. They killed one of ours!" - Ennis
A vengeful Ennis sought the aid of neighbouring packs including Talia Hale, Deucalion and Kali to strike back and left a revenge spiral carved into the wall of the abandoned distillery as a message, swearing revenge. When his beta's body was detained in the hospital morgue, Ennis demanded the corpse be released to him. He nearly beat up a hospital employee until he was calmed by Deputy Stilinski, saying they weren't related. Ennis talks back saying he was family to him.
Some time later, after the declaration of war upon the Packs instigated by Gerard Argent, Ennis was recruited by a corrupted Deucalion to form what would become the fearsome and dastardly Alpha Pack. Ennis accepted. He murdered the rest of his Pack, his Emissary to subsume all their power, and joined. Kali also accepted as she was involved with Ennis.
Just as he enters the elevator Ennis detects someone behind him. The teenaged werewolf Scott McCall makes eye contact with Ennis with a captive Isaac, identifying the older werewolf. Scott claws out, growls and rushes to protect his friend, but Ennis smirks, displaying his own claws, accepting the challenge. He allows Scott to rush into the doors.
Ennis: "Don't you realize what you're dealing with? I'm an alpha!"
The elevator doors opens and Derek Hale bursts in, having heard the fight. Derek instantly claws Ennis in the back rescuing the two Betas. Derek throws Ennis out of the elevator and the ousted alpha lands on the floor.
But they incapacitate her in the end and leave her to Deucalion.
"Ready for a rematch?"
The lead alpha however had anticipated this, having Ennis, Kali and the twins in turn converge on Derek's pack, catching them all off-guard, Ennis himself coming up from the market's lower level with his claws and fangs out. Derek makes the first move, attempting to take out Deucalion. Immediately, an all-out scrap sparks between the two packs.
Before Derek makes a decision, Allison Argent appears on an overhead ledge and fires flashbang arrows, blinding and distracting the far more formidable alphas turning the tide allowing Derek's pack to recover.
Ennis crashes into Scott, but suddenly, the teenager fends him off with equal strength. Scott's werewolf eyes are now an alpha's red, to Ennis' confusion. Unbeknownst to both, Deucalion catches notice of this, knowing Scott has potential to be a true alpha. Once Scott's eye colour changes back to gold, Ennis readies to attack again, but Derek attacks from behind with a sneak attack.
The two continue to fight, their scrap edging closer and closer to the edge of the supermarket's floor. Scott attempts to give Derek his aid, and slashes Ennis' calf from behind. This causes both Ennis and Derek who still have each other in their claws to topple over the edge. The two alphas fall two floors down, landing on a stairwell and are unconscious. Ennis and Derek are revealed to have survived the fall, but Ennis is severely injured.
Deucalion: "And the prognosis?"
Ennis: "Surprisingly optimistic. He's going to make it."
Right afterwards, the blind alpha crushes Ennis' skull, killing his subordinate alpha instead, as well as taking his power. Right afterward, Deucalion lies to his pack, saying that Ennis died from his injuries from fighting Derek. Kali lets a grieving roar and swears revenge unless Derek is to join their pack, as her treacherous leader intended.
After the alphas depart the clinic, Morrell covers Ennis' corpse with a white sheet.