In the Pilot, Costa is among the guardsmen removing the 100 prisoners from their cells on the Ark in readiness for their descent to the ground. Costa comes out of a cell to see that one of the prisoners, Clarke, is causing trouble and has momentarily eluded a couple of other guards. Costa moves to help apprehend Clarke but is told to wait by Abby, who is the Ark’s chief medical officer and Clarke’s mother. Costa keeps watch as Abby talks to her daughter and calms her down, before Clarke is shot in the back with a tranquiliser shot and subsequently sedated.
Chancellor Thelonious Jaha is badly wounded in an assassination attempt and Abby is arrested and sentenced to death for using too many medical supplies while saving his life. Costa is present at the airlock where Abby is to be “floated” and goes to grab her arm when Acting Chancellor Marcus Kane decrees that it is time for Abby to enter the airlock. Abby however, shakes Costa’s hand away, determined to walk to her fate without being indecently manhandled. Costa stands back and watches while Abby is about to be blasted into space when Jaha staggers down the hallway and orders the execution be stopped. Jaha, still recovering from his gunshot wound, puts a hand on Costa’s shoulder to steady himself and states that Abby is pardoned. Abby is then let out of the airlock by Shumway, the Ark Guard Commander.
In Contents Under Pressure, Costa aids Jaha in controlling the crowd which is about to attack Kane. Later he is standing sentry during the general assembly in which Jaha reveals the truth about the 100.
In Day Trip, Costa is one of the guards to assist Kane in arresting the traitorous Shumway.
When the Ark is brought to the ground Costa was on Mecca Station.
In The 48, Costa is one of the guardsmen with Kane when they kill the grounder leader Tristan and find Finn, Bellamy, Monroe, and Sterling. Costa then travels with Kane, Abby, Finn, Bellamy and 5 other guardsmen to investigate the status of the dropship. When they find Murphy and Raven, Costa helps carry Raven's stretcher. Once at Alpha station, Costa escorts Bellamy inside.
In Inclement Weather, Costa and Major Byrne bring Murphy into the holding cell. When they hear shots fired he runs outside with the others to investigate. Later, when they hear screaming, Costa follows Kane into the woods and helps cut down some guards who have been crucified by Grounders.
In Reapercussions, Costa is one of the guards to escort the captured Grounder Rivo through Camp Jaha and protect him from the mob. Later, he escorts Abby to her punishment and watches as she is shock lashed.
In Human Trials, Costa and Denby are the two guards helping Kane to escort Rivo to negotiate a truce with the Grounders. After the captured Grounder asks for Kane to untie him in Trigedasleng, Costa says that Rivo doesn't understand a word they're saying. Kane replies by saying that Rivo's apparent inability to speak English is just a tactic he's using so that the Arkers don't expect him to tell them anything. Once the group reaches the boundary where they must disarm before proceeding, Kane orders the two guards to return to camp, which they do so reluctantly.
Back in Camp Jaha, in Spacewalker, Costa is seen protecting Finn from the mob of Arkers who want to give him up to the grounders. He is one of two guardsmen who keep watch over Jaha and Raven in their makeshift cell and later, when Abby tells the Grounders they can't take Finn, Costa stands armed ready with Major Byrne and other guardsmen in anticipation of a Grounder attack. After Kane returns, Costa escorts Jaha to Abby to discuss their next move with Kane.
In Survival of the Fittest, Costa stands guard as Kane greets Indra and her warriors at Arkadia. Later when the Grounders are cooking their food, Costa is seen standing next to Kane.
In Bodyguard of Lies, after the acid fog has been disabled by Bellamy, Costa marches through the forest to the doors of Mount Weather with the Skaikru/Coalition Army.
In Blood Must Have Blood (Part 1), Costa helps another guard to move the drill which Monroe just finished using to bore through Mount Weather's main door. After Lexa betrays the Sky People and announces a retreat, Costa leaves with his people to head back to Arkadia.
In Blood Must Have Blood (Part 2), while heading back to Arkadia, Costa is captured by Mountain Men along with other Sky People who include Kane, Abby, Monroe and David, and sent to Level 5 to be harvested for bone marrow. After the Mountain Men are irradiated and the Arkadians are freed, Costa is seen carrying Abby's stretcher through the gates of the Alpha station settlement.
In Ye Who Enter Here, Costa is one of the guards escorting Kane and Acting Chancellor Abigail Griffin to Polis. Once at the city limits of Polis, the group dismount their vehicle and prepare to go ahead on foot once Indra's warriors have rendezvoused with them. Costa hooks a med kit to his belt but is told by Kane that he can't take it into the capital because it came from Mount Weather and would offend the Grounders. When in Polis, Costa continues to escort the Chancellor and Kane as well as taking in the sights and talking with the locals.
In Terms and Conditions, Costa is seen at the gates of Arkadia when the two Grounder messengers approach. He watches them converse with Pike and Bellamy and hears them issue a warning to the Arkadians that if they try to leave they will die. Costa watches as Bellamy kills both messengers.
In Join or Die, having been chipped by A.L.I.E., Costa is seen in the tower of Polis with another chipped guard when they deliver Abby to Kane (who has been imprisoned in the tower and is unaware that Abby is chipped). Costa and the chipped grounder warrior wait outside the door while Abby tries to get information from Kane regarding the whereabouts of Clarke and the Flame. As soon as Kane sees through the ruse and realizes Abby is chipped, Costa and his fellow guard enter the room and seize Kane – Abby orders the guards to put Kane on the cross.
Hauling Kane outside, Costa and several more of A.L.I.E.’s minions hold Kane down and chain him to the crucifixion cross. Costa then stands back as Briggs drives nails through Kane’s wrists and watches as Kane’s cross is heaved upright.
Costa is still keeping watch on Kane when Jaha and Ontari arrive. Jaha offers Kane the chip, but he refuses, so Costa gives Jaha his gun. Jaha threatens to shoot Abby in the head unless Kane takes A.L.I.E.’s chip, and so finally, Kane accepts the key.
In Echoes, After the City of Light has been destroyed, Major Costa brings a group of Sky People to the Flamekeeper's temple to lead them out of Polis through a secret tunnel. He meets Kane and tells him he has 10 guards at the temple entrance and offers to stay in the city. Kane declines his offer and tells Costa to lead the other Sky People back to Arkadia.
In The Tinder Box, Costa listens to David Miller's briefing on the defense of Arkadia from the impending assault of Azgeda. When Clarke arrives and suggests she goes to talk to King Roan, Monty alters her idea in favour of the Arkadians setting up ambush positions on the invading army's path.
When Roan's army attempts to pass through a canyon, Costa and the other Arkadian guards take up ambush positions on top of the adjacent cliffs. After Roan is persuaded to talk privately with Clarke, Costa and the other gunners continue to train their weapons on the Ice Nation army. While they are waiting, Monty radios Costa asking if he knows the whereabouts of Riley, one of the sharpshooters, Costa replies that Riley went to relive himself a short while ago. Monty asks if Riley took his rifle with him: glancing over at Riley's previous position, Costa notes that Riley did indeed take his weapon, but that he left his radio behind.
In DNR, Costa helps unload supplies from a truck returning from Becca's research lab alongside Nathan Miller and Eric Jackson. They approach Kane on the outskirts of Polis and the Chancellor tells them to wait while he goes to talk to Roan, who has been assisting the research team on Becca's Island. When Roan inquires as to where the guards are, Kane informs him that the Sky People aren’t back from Arkadia yet. Suddenly, Clarke warns Roan of an impending trap just as three Trikru warriors appear from the trees and move to attack Roan. Roan realizes Kane has betrayed him, but before he can react, the Trikru warriors are shot dead by Azgeda archers under Echo's command who have come to their King's aid. The Azgeda warriors move the capture all the Sky People and although Costa fights back and flings one of his assailants to the ground, he is eventually overcome and apprehended.
Costa is taken to Polis and imprisoned with the other Sky People, but he and the others are later released after it is agreed that a Final Conclave will be held to determine who gets control of a nuclear bunker recently found in the city.
In The Other Side, after Skaikru stole the bunker before the conclave ended, Costa is among those inside and stands guard at the door to the main office where Jaha is presiding. When Abby comes to speak to Jaha, Costa opens the door for her and lets her inside, before closing the door and resuming his position outside. A short while later though, Costa is either sedated or knocked out by Bellamy Blake, who is working with Abby to open the bunker's hatch in order to let his sister into the bunker (Octavia and Kane having been locked out while the conclave was still occurring). Clarke, realizing Abby and Bellamy’s plan, retrieves Costa’s sidearm from his prone body before pursuing Bellamy to the bunker’s main hatch.
In The Chosen, Costa was seen in the crowd in the bunker.