One night Ennis took his girlfriend Tamara to dinner. While both of them are having dinner, Ennis walks out of the dining table and asks the waiter to put an engagement ring in the champagne. However, when he was about to confront him, Ennis witnessed that something was wrong with his face in the mirror. Thinking that he was seeing things, he left the restaurant with his girlfriend and was about to propose when they both saw a wounded Sal walked out of the restaurant and is being pursued by someone. Ennis told Tamara to call the cops while he was trying to help Sal and listened to his last words. As Tamera was about to call the cops, the killer threw Tamara against a tin door, hitting her head. As a result, she got a severe wound and died at the scene. While Ennis was also thrown further, both Tamara and Sal were killed in front of him.
Next day, he was being interrogated by Detective Freddie Costa, who is on payroll for the Lassiter Family and thus disbelieves Ennis' version of events. Meanwhile, Sam and Dean enter the room as FBI agents and start talking to Ennis, throwing Freddie out of the room. Ennis says that he knows what he saw. On their way out, Sam and Dean tell him that there is no such things as monsters. Later, Ennis walks into his house and starts looking into his father's belongings, finding a lock pick, a revolver and a silver bullet under the gun.
He started looking in the separate pub inside the restaurant where the murder took place. As he enters, he sees a waiter that was removing all of the evidence. Ennis confronts him and tells him to surrender, but instead of surrendering to him, the waiter tried to feed on him. Sam and Dean arrive at the right moment and cut the monster's head off with a machete. They confess to Ennis that monsters do exist and gave him necessary tips and tricks to spot and kill them. However, they also warned him to stay away from this type of life.
Ennis was doing research into Sal Lassiter. Suddenly, Costa came to his home, asking the same questions he made earlier. To be sure that the officer is not a monster Ennis used a trick which he learned from the Winchester brothers. He saw through his cell phone camera that the officer's eyes are glowing. So Ennis took him to his room threw his jacket to his face and pointed a gun with a silver bullet and started interrogating him. The officer who was actually a shapeshifter, confessed to him that he was David Lassiter, Sal Lassiter's brother. He also said that a monster was not behind his brother's murder because the claws used on this jacket was silver and Julian Duval is a werewolf, silver works on almost every monster including werewolf. However, at some point David Lassiter managed to escape. Ennis used the name Julian Duval to find out if there is someone like him actually exists.
Ennis came to the Duval Family mansion where he saw Sam and Dean, waiting outside. Ennis stayed off their sights and trespassed inside the house when he heard a scream he rushed there as quickly as possible and saw that the person who killed his fiance is about to kill David, so he shot at him but he missed. Then he kidnapped Violet Duval, David's ex-girlfriend and sister to Julian Duval. When the Winchesters found them, they asked them what happened there? David said everything. There are five monster Family that runs Chicago. Dean who was both angry and surprised by this new piece of information, exclaimed "What is this, Godfather with fangs?" And Sam asked David that was his ex-girlfriend is a shapeshifter like him. When David answered that she was a werewolf, both Dean and Ennis said nothing but "Awesome!"
Sam and Dean found Violet's locations using the GPS signal of her mobile. David said to Ennis that he is sorry for his fiance's death and Ennis said that he is sorry for his brother's death and also said that his brother mentioned his name and said he was sorry.
When Sam and Ennis were looking for the killer, they saw David was captured by Irv Sokolowski. Sam, Dean and Ennis came found them when Irv was being mauled by Violet. Irv stood up and recognized Ennis. He apologized for Tamera's death and insisted that he should understand his actions, as monsters deserve to be killed. But Ennis said that he is the real monster and shot him.
When Sam and Dean returned Ennis to his home he wishes to be a hunter and asked them to teach him how to fight creatures like them. But Sam said the road he is trying to choose is dangerous and one way. Once he steps into this he can't go back to his normal life. When Sam and Dean left Ennis loaded the gun with silver bullets and started to investigate in Irv's hideout. Suddenly he got a phone call from someone telling him not to step into this path or else the monsters are going to kill him. Ennis recognized the voice from the phone. It was from his father.
Ennis Ross is described as "likeable, strong-willed, resolute, with a strong sense of justice". Ennis and Dean bear similarity in the way of talking except for Dean's comparing things with movie, songs or other things. He also bore some similarity to Sam in motives as Sam started hunting when his girlfriend was murdered. Ennis started hunting when his fiance was murdered. He also loved his fiance so much that he used to wear his fiance's proposal ring on his neck as necklace to remind how much he loved her.