On the Ark, Abby and Kane constantly had opposing views. Kane saw Abigail as a threat to his goals and frequently had her arrested and even almost floated her once due to his strict rule-following and her bending of the rules. Later, in The Calm, he seems to start caring for Abby, saving her life when he is exploring the other deck to look for survivors. On Earth, Marcus is the first person to follow Abby out of the Ark ship once it lands.
In The 48, they go out looking for the Camp of the Delinquents and Clarke together. After Abby gives guns to the Delinquents to help them escape and look for Clarke and the others in Inclement Weather, Marcus sentences her to shock-lashing in Reapercussions. Following, Marcus tells Abby he's leading a peace mission to get the kids back before passing along the Chancellorship to her. Marcus returns to Camp Jaha in Spacewalker and the two rejoice at the sight of each other. In Resurrection, Abby finds Marcus trapped under rubble after the missile hits Tondc. He is pulling on a piece of metal to make noise and she grabs his hand to stop him. Abby tries to help him and he tells her to leave him and go find Clarke until Abby explains that Clarke escaped as she already knew about the attack. When rocks fall down and trap them both further, they talk about how their actions on the Ark have influenced Clarke and the sins they have to answer for. In Blood Must Have Blood (Part 2), after they are captured by Mount Weather and Abby has her bone marrow extracted, they return to Camp Jaha, holding hands.
In Wanheda (Part 1), it is revealed that Abby is still looking for Clarke in the three months after the Mount Weather War, and Marcus is helping her, as she still holds the Chancellorship. However, in Watch the Thrones, she loses it to Charles Pike in an impromptu election demanded by Marcus when almost the whole camp votes for Pike instead of him. The two still remain a team, despite the new challenges that arise thanks to Chancellor Pike, for instance negotiating with the grounders who have been betrayed by Pike, and owing to the return of Jaha to Arkadia, with his new ideas about the City of Light. In Bitter Harvest, Abby kisses Marcus on the cheek, in order to give him "hope". They are seemingly separated until Stealing Fire. Marcus is to be executed along with Lincoln and Sinclair by Pike for his crimes against the Chancellor but the three men are freed by Harper, Miller, Bryan, Octavia, and Abby to leave Arkadia and escape the punishment. Abby, however, decides to stay behind. In response, he kisses her passionately and says, "May we meet again," to which she responds, "We will." The two then do not meet again until Join or Die, when Marcus goes to Polis to accompany Pike and survey the grounders' torture of him. Abby is there also, with Thelonious Jaha (as he and ALIE are chipping the grounders there, having chipped Ontari, their acting Heda, first). Marcus is thrown into a room in the tower, Abby entering shortly afterward and pretending to be unchipped in order to obtain Clarke's whereabouts from him. She seduces him but he soon discovers it's not her. Abby has him crucified for his resistance. However, Marcus' defiance wavers when Jaha threatens to kill Abby, and he takes the chip for her. Later, in Perverse Instantiation (Part 2), they do not reunite until the ending when Clarke shuts down the City of Light and all the chipped victims are saved. Abby and Marcus share an embrace while crying, to comfort each other.
Throughout Season One, they have many disputes and disagree on decisions that are made on the Ark. If it hadn't been for Thelonious Jaha, Marcus would have floated Abby for breaking the law and he seems pleased when Abby loses her seat in the Council to be replaced by Diana Sydney. However, following the Culling and the death of Marcus' mother, the two become closer and begin to understand each other's decisions. Abby comforts Marcus and he starts to take what Jaha told him to heart - that you need to know when not to follow the law.
Marcus is distressed when he thinks Abby was on the Exodus ship and is relieved when he finds she is alive. He is prepared to launch the Ark manually and sacrifice his own life and Abby tries to stop him. It is at this point Abby really starts to see how much Marcus has changed. However, Jaha is the one who launches the Ark manually and Abby and Marcus are sent to Earth.
In Season Two, their relationship is back to being rocky when Marcus, the acting Chancellor of Camp Jaha, says that they can't spare the men or weapons to send out a search party to look for Clarke and the rest of the missing delinquents who are trapped in Mount Weather. Abby goes against him and helps Bellamy, Finn, Monroe, Murphy and Sterling escape Camp Jaha. When Kane finds out, he knows that he must follow the advice of Major Byrne and punish her to restore order to Camp Jaha. He is forced to publicly shocklash her, even though he doesn't want to. After Abby has taken her punishment and Jackson is treating her in medical, Kane tells says that he "did hear [her]" and that he is leading a mission to make peace with the grounders and get their people back. Abby wants to go to, but Kane says she is needed at Camp. He makes her Chancellor before leaving, telling her that he trusts her to do what is best for their people.
In Spacewalker, When Kane returns to the Camp Jaha, Abby tells him she was afraid she would never see him again and he admits he feared the same thing. The two continue to argue about what is best for the Camp, but it is now clear to the audience that they have chemistry.
In Remember Me, Marcus reassures Abby that Clarke will overcome her grief and pain regarding Finn. He also tries to convince her to release Jaha but she brushes him off. Later, after Abby and Clarke have a disagreement, he approaches Clarke and tells her, "tearing each other and yourself apart isn't a way to get through this."
In Coup de Grâce, Abby expresses her worries over Clarke to Marcus who informs her that Clarke is strong. Abby tells him that the Grounders look at Clarke as the Skaikru leader and now, so does Mount Weather. Marcus then tells her that Clarke is the only thing holding the Skaikru-Coalition alliance together. Later, they have a disagreement regarding the treatment of the Mount Weather prisoner, Marcus wanting to torture him by exposing him to radiation and Abby deciding against it. However, when Clarke decides to free the prisoner so he can give a message to Mount Weather, Marcus tells Abby she should trust her daughter.
In Rubicon, Abby continues to express her doubts over Clarke's leadership towards Marcus, who attempts to reassure her. Abby then informs Marcus that Jaha and twelve others have left in search for the City of Light. Marcus tells her they can't afford to go after them. Abby tells him she knows and that Clarke wouldn't let do it even if she wanted to.
In Resurrection, Abby finds Marcus in the rubble and tries to save him. When more rubble falls on her and they start to fear they are going die, Abby stays strong for Marcus, who is badly injured. Marcus asks her to leave him and go and find Clarke, but Abby breaks and tells him that Clarke knew about the missile and that she blames herself because Clarke is her daughter and she feels responsible for her actions. They talk about the sins they must answer for and Abby questions whether they deserve to survive. Luckily, Octavia and the other seconds find them and save them from the wreckage. When the Grounder Army led by Clarke and Lexa marches on Mount Weather, Abby stays in Tondc to treat Marcus and the wounded.
In Blood Must Have Blood (Part 2), Marcus and Abby are captured along with Monroe and David Miller and are going to be killed for bone marrow. When Clarke kills Dante Wallace, Cage Wallace puts Abby on the operating table and is prepared to kill her. Marcus is extremely distressed and says that they can donate bone marrow and that nobody has to die. When level 5 is irradiated, the first thing Marcus does is free Abby. The last we see of them in season 2 is when they return Camp Jaha. Abby is on a stretcher and the two are holding hands.
In Season Three, Marcus and Abby's relationship has mellowed and softened. It is clear that they now trust each other completely, rely on each other, and look to each other for guidance and judgment.
In Wanheda (Part 1), it becomes apparent that Marcus has been looking for Clarke just as much as Abby has, and in fact is more of a co-leader with her than simply a Councilor, helping her to make most critical decisions in her Chancellorship.
In Ye Who Enter Here, they mildly argue about Abby's choice to move an exploratory group into the old Mount Weather facility for the medical supplies, with Marcus saying it's too dangerous a move. This raises further discussion, as Marcus says that "takes away from everything else [they've] been negotiating for", including lifting Lincoln's kill order, opening trade routes and getting Clarke back. He immediately apologizes for using Clarke as a bargaining chip and reassures Abby "they won't leave Polis without [her] daughter". They explore the city together and Abby discovers he has a way with people, and a natural talent for being a leader. She wants to pass on the Chancellorship right away but Marcus makes her wait until they can get back to Arkadia and have a proper election so the people can decide. Yet later, when the grounders ask who is the Chancellor to brand them with the mark of the Coalition to become the thirteenth clan, Abby pushes Marcus to do it.
In Watch the Thrones, Marcus and Abby are shown to be a united force against Pike, though Abby loses the Chancellorship to him.
In Hakeldama, Marcus and Abby continue to work together against both Jaha and Pike though Marcus loses his job on the guard. They then both help Octavia and Clarke escape Arkadia.
In Bitter Harvest, Abby kisses Marcus on the cheek to give them both 'hope', as she and Jackson had called it.
In Stealing Fire, Marcus is given the death sentence for his crimes of betrayal against the acting Chancellor (Pike). The first time they say goodbye, Abby reveals her affection for Marcus when she cradles his face in her hands and tells him, "I can't do this again," in reference to her late husband Jake. Marcus stops her from saving him because he knows she'll be condemned to death, too. The second time they say their goodbyes, Marcus is about to escape when he kisses Abby and tells her "May we meet again."
In Join or Die, the two of them are reunited though Abby is chipped and has Marcus nailed to a cross until he submits to the chip when her life is threatened.
In Echoes, Abby and Kane come down from the Polis tower together. Later, in the temple, Abby checks Kane's crucifixion wounds. Kane caresses her cheek and reassures her he's ok and they need to focus on what comes next. At the end of the episode, when Bellamy and Clarke leave Polis, Abby and Kane stay behind together.
In Heavy Lies the Crown, Abby and Kane are seen after sleeping together. Abby tells Kane that Roan is expecting her and he helps her fasten on her necklace with Jake's ring. Later, Abby tells Kane she is no longer needed in Polis by Roan and she needs to return to Arkadia. Kane notices she's no longer wearing her necklace with Jake's ring and they kiss.
Kane: "No, no I haven't. I let this happen. I demanded an election, and now we have Chancellor Pike. We're on the brink of a war we can't win, and all I can do to stop it is send kids into harm's way."
Abby: "Hey, in case you haven't noticed, they're not kids anymore."
Kane: "With Clarke gone, Bellamy's the key. I've been trying to reach him, but Pike's influence is strong, and he really believes he's doing the right thing."