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 so to establish perfect power. Ennis accepted the offer, killing all his betas and his emissary to take their power, augmenting his alpha status to join, alongside fellow alpha Kali whom he became romantically involved with. In Season 3, Ennis, as part of the Alpha Pack, arrives in Beacon Hills having taken notice of Derek'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 purposes, 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 wolves 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 is 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, oblivious teenager, Paige Krasikeva, viciously Biting her and 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 killed them by accident. Ennis' Wolf was later hunted down then killed by the Argents.
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.
The Alpha Pack infiltrate the hospital to capture their targets, beta werewolf Isaac Lahey and the mercenary called Braeden. Isaac has been wheeled into surgery but is later taken out because of his healing factor taking care of his wounds. Ennis, disguised as a nurse, rolls Isaac on the gurney out of the operating room. He then wheelchairs the unconscious werewolf towards the elevator.
Scott claws out, growls and rushes to help his friend, but Ennis smirks, displaying his own claws, accepting the challenge. He allows Scott to rush into the doors. Once the elevator closes, Ennis fully shifts, unmasking his heightened alpha power, to which Scott pulls back slightly. Ennis then beats Scott up in the scrap. He lifts Scott up by the throat with one hand, gloating and threatening him.
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: "So am I."
Derek throws Ennis out of the elevator and the ousted alpha lands on the floor before some people, but the alpha shifts back in time.
Ennis appears in the high school locker room with the other four alphas. They all take on and overpower the mercenary Braeden.
"Ready for a rematch?"
Deucalion motions at Ennis to release Cora as a gesture of good will. Ennis stands silent as Deucalion gives his proposition to Derek telling the beaten Alpha he, Ennis himself and Kali have all killed their previous Packs, absorbing their power to form the Alpha Pack.
The Alphas have the Pack dominated, but Allison appears on an overhead ledge. She fires flashbang arrows, blinding and distracting the far more formidable Alphas turning the tide allowing Derek's Pack to recover. Scott picks himself up and faces off against Ennis. Ennis charges the younger, lesser Wolf. Ennis crashes into Scott, but inexplicably, all of a sudden, Scott fends off the Alpha with equal strength (which Deucalion inconspicuously catches). Scott's Wolf eyes are Alpha red, to Ennis' confusion. Once Scott's eye colour changes back to bright gold, Enis readies again, but Derek takes him on from behind. While the two Alphas grabble, Scott slashes Ennis' calf from behind, causing Ennis with Derek in his grasp to stumble over a ledge opening, falling two floors down, landing on a stairwell.
Ennis and Derek survive the fall, but Ennis is severely injured. Kali and Aiden, along with Marin Morrell, bring Ennis to the animal clinic to get treatment from Alan Deaton. Alan is unwilling to help the dastardly Pack, but relents once Marin makes it clear they'll kill Scott.
Deucalion's intention for killing Ennis, and framing Derek, was due to the Alpha Pack's own rules stating that Derek must now either join the Pack and kill his own in penance for Ennis' death, or they will kill him. He was intending to incite a distraught, vengeful Kali to force Derek's decision to join as part of his plan to place Scott and Derek against one another, so to pave the way for Scott joining the Pack.