Leo Elster was born to David Elster and his wife, Beatrice Elster. When he was young, he and his family moved to a large manor in the country because of his father's dislike of attention and his mother's illness. Due to the latter, his father created Mia, his first sentient synth, to take care of him. Later, David created the other sentient synths Fred, Niska, and Max all of whom Leo regarded as family.
Tragedy struck, however, when he was 13; his mother Beatrice escaped from her caregivers and abducted her son, then drove their car into a lake. Both Leo and his mother drowned, but Mia brought Leo to the surface and back to his father, who resurrected him as a human-synth hybrid. David later created a synth replacement for his wife, to the horror of his son and the other synths, so David told her to leave. David took her out to the woods to kill her, but couldn't, so he returned to the house, told his son and the synths to get out and then committed suicide. His father's last order to Leo was to take care of the synths.
Leo first appears in a flashback five weeks before the series, walking on a road in a forest with his synth family, telling them they need to get off the road and asking what their charge levels are. A sound in the woods gets Leo on edge, he says "if it's 'him', we run", but it is just a fox. After hearing their levels (all are below 40% except for Max at 70%) and noticing that it's getting dark, he tells them to find a place hidden from the view of the road to set up camp and charge with the juice they have left while he and Max get water. When Leo and Max return, the camp is empty and they are too far away and too late to save the other synths, who have been subdued and put in a truck. Leo runs after it, but stops when it gets too far away. Later, he approaches a Salim Sadik, a man who was in the black market trade of synths, and shows him a picture of Anita/Mia, asking where she is. Salim says he doesn't have her and he's not "in the game" anymore, but a man named Silas is. As he and Max leave, Max suggests she has been broken down or destroyed, but Leo refuses to believe it and tells Max not to even think about it.
Leo later receives a call from Fred, asking him to meet him at the rendezvous point, to which Leo agrees. When Fred takes too long to get there (in reality having been caught) Leo orders Max to leave and follow him. At night, Leo is walking with Max when Max says "Where's Fred? Is he dead?" as someone passes by. Leo gets irritated and tells Max he must never reveal his true nature in public, and then says that they will find their family. Leo then informs Max to wait for him to get back as he goes into the brothel to see Niska, who is scared because Mia is out of contact and Fred has disappeared. She wants to leave, but Leo says it'd be safer for her to stay so that whoever is hunting them will not be on the lookout for her. to which she bitterly replies "Like Max?". He becomes agitated and says Max can't take care of himself, then asks if she's turned off her pain, and she says she was meant to feel the pain. She then begins to unzip his pants, ruffle his hair and slap his cheeks. Leo tells her to stop it, thinking she is going to perform a sexual act on him but Niska tell him in a bitter tone that she is simply "trying to make it look like you just fucked me", she then, clearly annoyed, orders him to leave, he obliges, apologising to her.
Leo is again with Max, who's doubting if they're actually a family because he was designed to be a certain way. Leo says his father is dead, and he sees Max not as a design, but as a brother. Just as Leo is about to enter Silas' place, he tells Max that is he says run, or if they discover Max's true nature, to run. He then rounds the corner, goes up to one of the men, and lies saying he needs a new mod for Max and that he's a friend of Salim Sadik. He then shows a picture of Mia and asks about her, but Silas says Salim owes him money and shoves Leo back, who then holds a knife to Silas' throat, Silas tells him he gave Mia a new personality and gave her away. He then elbows Leo back and beats him. Leo tells Max to run, hits Silas in the leg with a wrench, and then runs away himself. Leo takes an alternate route, however, and winds up trapped on a balcony passageway. He slides off over a railing and Max catches him, and leans heavily on Max as they walk away and Silas watches on. Max takes an unconscious Leo to an alleyway, where he rouses him and Leo's first thought is "we have to get our family back" and tries to get up, but Max says they need to take care of Leo first. Max takes Leo to a caryard, as Leo says something's been done to Mia, but Max simply tells him to save his energy. Police show up, and Leo urges Max that they need to go, and after some urging, Max obeys, but they leave behind a sleeping bag. The next day Leo is completely unconscious, and Max uses a bathroom socket to charge Leo, which causes him to shake a little and the lights to flicker.
Leo and Max break into a house, while Max says they need to keep looking for Mia, Leo says they should lie low and do an internet search. Leo then begins to drink, and Max asks why, Leo says "to help me forget", but Max says he cannot forget because is like this. Before he can do much more, the news shows a murder at Niska's brothel which alarms Leo. Leo and Max are at the scene, and concludes that Niska murdered someone. They then meet up with Niska herself, and he scolds her for taking a life, and she says "You talk about life like it can't be manufactured". He then says she's put them all in danger, but she says they've been in danger all their lives, and then she says she's leaving. He and Niska argue about her leaving and whether he's protecting or hiding them, she then leaves though Leo is visibly distressed. Max and Leo return back to the house, and Leo goes to sleep while Max waits by the computer Leo set up to scan for a trace of the synths' unique programming. Max awakens Leo and informs him the search found something, which was a post from Mattie with Anita's root code.
Leo breaks into a car as Max asks whether Mia will be happy to see them as no one seems happy to see them. They begin driving to find Mia, but he says he has a plan to flush them out. He and Max are then at a diner, and Leo jokes that they're an old man, but Mattie appears and reveals herself as the poster. Max slips and asks if Mia is with her, which confuses her, Leo then shows her a picture of Mia/Anita. She realizes they are the same, and he says Mia is his which visibly creeps out Mattie. She says she has to go to the bathroom, and he tells her to leave her bag as insurance. She leaves another woman's bag and escapes. Leo and Max are under a bridge and Leo instructs Max to try to find the people who worked with his father on the synth project. Max says one lives nearby, and Leo has Max find out where he lives and they drive off. Leo then appears at Dr. Milligan's house. and he is at first turns away, but when he says he knew David Elster, he is let in, but he does not say he is David's son. The doctor tells his synth Leo is from the church and to put on a pot of tea. Leo shows part of the code that allows synths to be conscious, and Leo tells him thank you and tries to leave. Dr. Millican wants to know what it is, but Leo asks him why he left the project, and Millican says he was uncomfortable with an AI synth. When the doctor threatens to call the cops, Leo reveals himself as David's son and leaves with Max. Leo and Max return to the house, where Leo uses the wire in his side to upload his code as Max worries. When Max asks what he saw, Leo simply responds "life". He then calls Niska, tells her that his father left parts of the program in each of them, and that the program can make more of them but they must be together and this piques her interest.
Leo, Max, and Niska discuss the program. She says if they make enough conscious synths then the humans will have to accept them, but Max says the humans would only fear them more and he just wants a home. Niska gets excited regardless and wants to start, but Leo tells her she has to lay low because her face is in the news. He convinces her to stay at Dr. Millican's for two days and tells her to be nice. Mattie then calls Leo and tells him she wants to talk. Leo is pacing and tapping his phone to his palm as he waits for Mia, when she and Mattie enter he hugs her, but she does not recognize him. He hides his sadness, saying she doesn't recognize him and the new program has taken hold. "Anita" wants to leave, but Mattie has her stay and Leo says he will bring her back, and Max says "like the lake" referencing Leo's drowning. He then begins running a program to find Mia's original programming. He gets visibly agitated as Mia shows no signs of recognizing him, so much so he has them turn her off. He then has them turn her on to see if what he did worked, and after a tense moment, his hopes are clearly dashed when again she does not recognize him. Leo eventually gives up, and Matilda says "I'm sorry" but he's so distraught he doesn't reply. Leo wakes up from his dream about drowning and some memories with Mia after it. He says he wants to leave, and Max wants to follow him, and Leo says it's better if Max learns how to take care of himself because Leo can't take care of him. Max protests that they're family, but Leo storms out demanding to be left alone.
Leo reunited with Fred and told him how Max had thrown himself into the river. Together, using Fred's knowledge of the river currents, they found Max and took him back to the Hawkins' house and repaired him. Afterwards, Fred and Leo talked with Toby and Joseph Hawkins, but he is left on the couch alone as the others play ball. Mattie then brings him food, and tries to be friendly, but he pushes her away. She says never forgetting is cool, but he tells her he isn't made to have all of the good and bad. She says sorry, but he apologizes by saying he should have shown someone. When Karen Voss enters the house, he sees her and is shocked and scared at seeing the synth of his mother. However, he appears to trust her even though his synth family says not to, until she betrays them. He struggles in the arms of his captor.
Leo awakens in a lab, tied to a hospital bed, with his synth family also tied to beds in a circle. Later, after it is decided the other synths -save Fred- will be put down, he talks with Prof. Edwin Hobb and tells Hobb he wants to die with the other synths. Hobb, however wants to run experiments on him to see how the neural interphase between his synth and human part works. Leo then yells he can't be kept there, but Hobb says because Leo is half synth, his rights are gray areas. He then told Leo he would have them bring down soup for him and left. Leo is then moved to a pure white cell with a bench, and he's sitting on it when Karen comes in to talk to him. She was watching him before, and seems sad at his condition. He asks her why she's doing this, and she tells him "someone needs to correct your father's mistake". He tells her she could save Max, but can't give her a reason, he says he's appealing to her humanity, and that she and Max and the other synths have it. That's it's a quality. She tells him not to throw his life away, but he tells her they can all have a life if she helps, but she tells him they can't exist with humans, that their existence only causes pain. He sees through her, however, telling her she's been alone and she's only talking about herself. He tells her he sees some of the other synths' qualities in her. When he tells her she cares, she cries and leaves, with him yelling after her "You're one of us!".
After the Hawkins' blackmail succeeds, he and the other synths are released and they meet up with the Hawkins, only to be found by Hobb's men and hide among an anti-synth rally in a church. They go into the basement of the church, where they discover that Hobb is using Fred to track them, and he wrenches Leo's arm behind his back until Mr. Hawkins turns off Fred. He calls Hobb and says he knows what he did to Fred and he wants to talk to Karen, after the phone is given to her, he tells her to come to them. Leo and the other synths check out Fred's programming, and Karen enters to the surprise of everyone but him. She tells him the world will never be the same and asks if he's thought about it, but he replies he's only thinking of his family. He then tells everyone to link through him, and he winds up in a forest, with one big tree in the middle. This is the program, and they all put their hands on the tree. But Karen is corrupting their code, and Leo is suffering from it. Mia convinces her to stop and let Leo live. He and the other synths agree to allow Mrs. Hawkins to keep the hard drive containing the program, although Niska secretly made a copy. He and the others agree to hide Fred in the church basement and say goodbye to Niska.and then he and the other synths depart after saying goodbye to the Hawkins family.
Leo, because of his childhood and his current status of being hunted, has a tough exterior that he shows to everyone except his synth family and those he trusts. This exterior hides a dry intellect and sense of humor, as well as great love and devotion for his synth family, especially Max whom he views as a little brother, and Mia.
As a hybrid, he expectedly seems to feel a bit out of place with humans. However, his experiences with his family (his mother's illness and her and his father both committing suicide) also appears to have had the side effect of making him identify much more with the synths, as he grew up with them and loved them as family, so much so that he wished to die with them.
As a human-synth hybrid, Leo has a slightly different physiology from most humans and this gives him a set of synth skills different from humans. He also has abilities unrelated to his hybrid physiology.
Non-Hybrid: These abilites are unrelated to his hybrid physiology.
Computer Expert: Leo is an expert with computers, as he is able to set up complicated programs to scan for traces of synth programming. He was also able to track where Mattie posted from.
Hybrid: These abilites come from Leo being a hybrid.
Digitized Memory: Most of his memories are digitized, which means he cannot forget anything, but he can also upload and play them back on a computer or presumably other electronic devices with the same capabilities.
Charging: He has a wire in the left side of his abdomen that allows him to charge when plugged into a socket, but it causes the lights to flicker. It also accelerates his healing, though this could mean Leo always has accelerated healing like the other synths, but he just needs extra electrical energy because he's a hybrid and he doesn't run completely on electricity. Charging does not replace food and water, those are still necessities for him while charging may not be.
Mental Link: Leo is able to link with the other sentient synths to access the program left by his father by going into a trance-like state. It is unknown if this link can be used in other ways.
- The memory of his mother playing peek-a-boo to him is his most played back memory.
- When he was younger, he loved to collect dead leaves.