Robin

Prelude
In the original timeline, after Robin and Chrom defeat Validar at The Dragon's Table, Robin becomes possessed by Grima and kills Chrom. Robin's consciousness is completely taken over by Grima in this timeline, who uses her life force to channel his powers through her body. When Lucina and the other children attempt to escape to the past, Grima follows her to stop her. Upon returning to the present, Grima tried to fill the present Robin with his memories; however, Robin's body was not strong enough to retain the memories, weakening Grima's power. Robin developed amnesia as a result, however she saved her name and her skill with war tactics. Because Grima poured his memories into Robin, she occasionally are flashed with her future version's memories from time to time, resulting in migraines.

Ylisse-Plegia War
At the beginning of the game, Robin is lying unconscious on a field, when she is rescued by Chrom's Shepherds. Despite losing her memory, Robin has great knowledge of fighting battles and serves as their tactician. Leading Chrom and the Shepherds through a long campaign to protect Ylisse from the Plegians, Robin marries Chrom, and she becomes the new Queen of Ylisse and gives birth to Lucina two years later.

Conquest of Valm
Two years after Gangrel's defeat, Valm intends to invade Ylisse. Chrom and Robin decide to talk to the new king of Plegia who is none other than Validar, who was slain in Emmeryn's assassination attempt. Validar also introduces his hierophant, who looks exactly like Robin, and even introduces herself by the same name. Nevertheless, Validar gives Chrom and Robin the necessary provisions to go to war. Later that night, a voice calls to Robin. Validar appears and reveals that Robin is the descendant of a long line of Grimleal leaders and the first one who had all the markings to become his vessel. However, overcome with emotions, Robin's mother took her from her crib and fled Plegia altogether.

Before Validar can further try to sway Robin into joining him, he is forced to flee when Chrom arrives. Robin shakes off this event as the Risen attack Chrom's army. After nearly fending off all the Risen, one manages to warp in behind Chrom and nearly kills Lissa, but Marth suddenly enters the battlefield and saves them. After Marth reveals that she is Lucina, she shares a moment with Chrom, Robin, and Lissa where she tells them about the future which she came from: a future where everyone in the army dies. Lucina share a tender reunion with her mother. Despite this new development, Chrom and Robin prepare to travel across Valm to stop Walhart. Occasionally Robin is stricken with migraines, but she ignores these headaches and focuses on her goal. After a long campaign, Robin leads Chrom's army to victory over Walhart and the Valmese forces. By the end of the war, they had gathered four of the Emblem's five gemstones.

Fate of the World
After taking down Walhart, Chrom and Robin receive news that Validar is going to give them the final gemstone for the Fire Emblem, but Validar betrays Chrom and tries to take the Fire Emblem from Chrom. Robin, Chrom, and Lucina attempt to flee the castle, but right when they are about to escape, Validar warps in and attacks Chrom. Validar orders Robin to take the Fire Emblem from Chrom, which she does against her will, and gives it to Validar, who disappears after taking the Emblem. Later, Lucina decides to talk to Robin privately. After watching Validar control Robin, she deduces that Robin killed Chrom against her will. Lucina was going to kill Robin but is conflicted and cannot bring herself to kill her own mother.

Before confronting Validar at The Dragon's Table, Robin makes Chrom promise to be ready to kill her, in case Validar controls her yet again, which Chrom reluctantly agrees to. Upon reaching Validar in The Dragon's Table, Validar reveals that Robin was born to be the vessel for the Fell Dragon Grima, because Robin possesses the Heart of Grima. Robin bears the Mark of Grima on her right hand, a symbol of her role as Grima's vessel. After defeating Validar, it appears that Robin is "possessed" by Grima yet again and "kills" Chrom. However this is a ruse, revealing that due to a Premonition, she knew what was going to happen, so they prepared countermeasures to prevent this. Robin managed to hold back on her power, saving Chrom from any fatal injuries. In the continuing battle, the duo proceed to cut Validar down.

After the battle, a mysterious black figure warps in and takes on the appearance of Robin. She explains that she is Grima and possessed Robin from Lucina's future. Robin questions Future Grima, asking if they are not yet the same. Taking that question as Robin's refusal to become Grima's vessel, Future Grima decides that it is not necessary, since she can perform the ritual again herselfs, and proceeds to awaken the present Grima. As Chrom and Lucina flee the building, Robin quickly retrieves the Emblem herself from Validar's corpse. With the Emblem in hand, the Shepherds depart to Mount Prism in order for Chrom to complete the Awakening Ritual to finish off Grima once and for all. Chrom completes the trial and unlocks the true power of the Falchion. Naga informs Chrom and Robin that the Falchion, even with her blessing, can only put him into a 1000 year slumber. The only way for Grima to die is for him to "be killed by his own hand." Naga points them to Origin Peak to confront Grima. Upon arriving at the peak, Robin realizes that, since she was meant to be Grima's vessel, she has the power to slay him. However, Naga warns her that if she does not kill Grima, she will perish, too, since her heart is inexorably connected to Grima's. Chrom refuses to let Robin sacrifice herself, but she still considers going through it.

The Shepherds get onto Grima's back, but are instantly struck by a spell that severely weakens the army. Grima gives Robin the choice to take the power of Grima or perish. Robin chooses to perish, Grima pulls her into a dark void and tells her after merging that he will kill Chrom's army and severely weakens Robin. Out of ideas and hope, Robin lies in despair. However, a sound pierces the darkness: the voices of Chrom's army, telling her to stand back up and fight. To Grima's shock, Robin breaks free from the void and rejoins the Shepherds. After Naga fully heals the army from the Fell Dragon's magic, they proceed to face Grima.

At the end of the battle Robin lets Chrom finish Grima, Grima goes back into a deep slumber, only to awaken at a later time in the distant future. While feeling guilty for only delaying Grima's destruction, Robin's family consoles her and lets her know that she made the right choice. Chrom also reminds her that even in the future, humanity will be more than ready to face the Fell Dragon, much like they just did.