She was portrayed by Tasya Teles.
Echo was a loyal warrior and spy for the Azgeda tribe. She was a dangerous enemy as she distrusted the Skaikru and tried to convince King Roan to break the alliance with them. However, Echo switched sides and redeemed herself from her mistakes by fighting alongside Clarke and her friends for the rest of the series.
In the past, she was trained by Queen Nia of the Ice Nation, to become a fighter and spy, forcing her to kill her friend. This hardened her and her name was changed from Ash to Echo. It hardened the girl who had become cold, murderous and true to her cause.
In season two, Echo is one of the prisoners of Mount Weather until she allies with Bellamy to successfully escape everyone. After which, she returned to Azgeda and conspired to manipulate Skaikru and detonate Mount Weather, under Nia's orders.
For this, she is taken prisoner by guards of Sky People and released by Bellamy. She tells him that an assassin is about to kill the Sky People delegation and guides him, Pike and Octavia through the Polis tunnels to warn everyone and Echo disappears. It was in fact a diversion where an assassin was at Mount Weather which he detonated, and this thanks to the cooperation of Carl Emerson, the last survivor of this place. She was chipped off-screen and watched Clarke defeat A.L.I.E., freeing all of the chipped ones.
When the AI is defeated, Echo blames Skaikru for the chaos caused with A.L.I.E. and immediately becomes suspicious of them. Despite Echo's hindrances, Clarke and Abby heal Roan who becomes the ruler of Azgeda and Echo bow to him. She tries to convince him to break the alliance established by Lexa and when she sees Skaikru rebuilding the Alpha Station to survive the Praimfaya, she takes the opportunity to interpret it as betrayal. Roan has no choice but to declare war, to Echo's satisfaction.
After which, she pursues Octavia with two guards and leads a fight against her in which she visibly kills Octavia who falls off the cliff. She breaks the news to Bellamy and Kane and seems uncomfortable about what she has done. Later, a conflict is caused by the clans to live in the bunker and a conclave is organized in which Roan will fight for Azgeda.
During the battle, Echo, in hiding kills Ilian but is arrested when Bellamy fights her and she is banished by Roan for her cheating, telling him that she will have no room in the bunker. She is then shocked to learn that Roan is dead. Eventually, she will go to space for six years aboard the Ring with Bellamy, Raven, Murphy, Monty, Harper and Emori.
In season five, Echo got closer to Bellamy and she blames herself for her past crimes, although her new friends now trust her. Upon returning to Earth, she learns of the arrival of the Eligius Prisoners led by Paxton McCreary and Charmaine Diyoza who fight against Wonkru to obtain Shallow Valley. Echo will help defeat McCreary's group with Diyoza and Shaw despite her distrust of them. The battle is successful, and Wonkru, Spacekru, and the surviving Prisoners leave Earth after McCreary fires missiles, wiping out the valley.
At the start of season six, Echo wakes up with his friends to listen to Monty and Harper's recordings of the discovery of a new habitable planet. She goes there and undergoes hallucinations due to the toxin of the Red Sun. After which she and the others meet the citizens of Sanctum led by Russell Lightbourne. Like them, she later discovers that the Primes kill other humans to make them hosts and transfer their spirits to this body to live forever. While attempting an uprising, Echo is arrested by Russell who wants to make her a host for his wife Simone. She is then taken to Ryker Desai who will take care of changing her blood into Nightblood. In his studio, she tells him about her past where she had to kill her friend and became Echo. The engineer hesitates then decides to inject the blood anyway, but thanks to the intervention of Gaia and Miller, Echo frees herself and kill Ryker. After which, she hides with them in a tavern and finds Clarke who is no longer in Josephine's possession.
In season seven, Echo and Gabriel see Bellamy being kidnapped by unseen people. Despite the hallucinations where she sees Roan calling her a spy, Echo manages to kill one of the kidnappers and goes in search of Bellamy with Gabriel and Hope whom she has just met.
With them, she arrives on Skyring where he meets Orlando, a banished Disciple who will live with them a few years before they leave him there to find Bellamy on Bardo. There, Echo lets his rage kill guards to the shock of Gabriel who tells him that it is not necessary and that they do not have time. They then reunite with Octavia and learn of Bellamy's apparent death, much to Echo's shock.
After being reunited with Diyoza, Clarke, and her group, Echo grabs the Gen-9, a genocidal weapon that she prepares to pour into a conduit to avenge Bellamy's death. Thanks to Levitt who alerted them, his friends run to see her and it is finally Hope, killing Anders, who tries to spill the liquid but Diyoza sacrifices himself when the drop falls on his hand and crystallizes the whole room. Despite the incident, Bill Cadogan lodges the group if Clarke gives him the Code of Flame to start the Last War. Eventually, Cadogan is defeated and Clarke passes the test that allows all humans to transcend and live in peace. However, she did not transcend and reunite with Echo and the rest of her surviving friends on Earth.