One night, Byleth and their father Jeralt rescue three young nobles, Edelgard, Dimitri, and Claude from bandits. Byleth nearly takes a mortal blow for Edelgard while trying to project her and is only saved by Sothis' ability to reverse time and have them redo their actions. After the bandit attack Jeralt and Byleth are then summoned to Garreg Mach Monastery, where Byleth is made a professor of the monastery's Officers Academy.
At the Academy Byleth has the choice to become the professor of one of the houses present, including the Black Eagles which Edelgard is the house leader of. Before choosing a house Edelgard will note the similarities between herself and Byleth, in that they both appear to be stand-offish.
Black Eagles Route
After Byleth joins the Black Eagles they can slowly become closer to Edelgard as she begins to open up about her past and her destain for the current system of nobility and its use of crests.
Later during the Ethereal Moon a ball is held and Byleth can meet Edelgard at the Goddess Tower, which has been fabled as good luck for couples. Edelgard will tell Byleth how the story surrounding the Goddess Tower is actual based on her father who met her mother there while he was attending the Academy.
If Byleth and Edelgard are close enough by the Pegasus Moon then Edelgard will ask for them to return home with her so that Byleth can witness an important ceremony. If they agree then they are able to see Edelgard become the new Emperor. Soon after the Black Eagles embark on a mission with Rhea to the Holy Tomb. Once there, it is revealed that Edelgard is the Flame Emperor and intends to take the crest stones within the tome and start a war against the church. Enraged, Rhea will demand that Byleth kill Edelgard. However, if Byleth was present for Edelgard's coronation then they will be have option to "protect Edelgard", siding with her against the Rhea and the church.
A battle at the Academy quickly ensues as Edelgard captures Garreg Mach Monastery. In the aftermath, Rhea transforms into her dragon form and buries Byleth under rubble as she escapes. Byleth is not found for five years.
Time Skip: Crimson Flower
After having "died" at the battle of Garreg Mach Monastery, Byleth is awoken by Sothis and returns to the academy to be reunited with their students. Edelgard is the first person Byleth sees upon their return and after momentarily being shocked that they are back, Edelgard quickly hugs them, telling Byleth that she was heart-broken after their disappearance and couldn't find them no matter how hard she searched, but never gave up hope that they would return. In the reunion with the rest of the Black Eagles, their classmates will bring up how much of an anchor Byleth was to Edelgard, and Caspar mentions that Byleth's disappearance broke her.
Byleth joins Edelgard in the war effort as part of the Black Eagle Strike Force. They continue to work together to take on the Alliance and The Kingdom, before going up against their ultimate goal: removing Rhea and the church from power. In the final battle, Rhea refuses Edelgard's call for surrender, and instead sets fire to Fhirdiad, forcing the Black Eagle Strike Force to attack and put a stop to her. Working together, Byleth and Edelgard are able to take down Rhea who is killed in the aftermath. However, Byleth collapses soon afterwards from their choice to defy the fate laid out for them, severing their connection to the Goddess. Edelgard tries to listen for Byleth's heartbeat but after hearing none, she begins to cry. As this happens the Crest Stone embedded in Byleth's heart dissolves, giving them a heartbeat for the first time. Upon hearing it Edelgard hugs Byleth once more, happy and relieved that they're alive.
After the war is won, Byleth can use the wedding ring their father gave them to propose to Edelgard. In their localized paired ending it is said that they "were sometimes spotted leaving the palace to privately enjoy the world they created. How they spent those precious moments, none but the two of them will ever know." In their original paired ending, it is said that "At times, the two left the palace to go on intimate honeymoon trips together. However, how they spent those moments loving each other is unknown to everyone but them."
- In Chapter 10: Where the Goddess Dwells if Byleth is female then Edelgard will carry them back to camp after they faint, otherwise she will get Hubert to carry them.
- As the story progresses and they get closer, Byleth will start teasing Edelgard who is the only lord character to receive this treatment.
- One of Edelgard's voiced lines in Fire Emblem Heroes references her attempts to paint a portrait of Byleth.
While Edelgard can be romanced by either a male of female Byleth, the ship tends to be popular when paired with a female Byleth. She is also one of the few characters to show to have explicitly romantic feelings towards Byleth in the game outside supports.
On AO3, it is the most written femslash ship for the Three Houses fandom and the most written ship for Edelgard herself.
- Edelgard is one of the seven characters who can have a romantic S-Supports with both a male and a female Byleth.
- Edelgard is the first lord, excluding avatars, that can explicity romance characters of both genders.