Before Kingdom Hearts: Dark Road

• Ymir and Xehanort meet when he appears in Scala ad Caelum - the two are drawn to each other, but they aren't really sure why.
• The two start to spend more time together when Xehanort befriends Eraqus, who is already friends with Ymir.
• Xehanort offers to help Ymir learn to use her keyblade more effectively, as external pressures make it difficult for her to have the confidence to harness the power of her heart correctly. They meet up frequently after school to do this, and during this time is where they start to develop romantic feelings for each other.
• Eventually with the extra practice, Ymir is able to channel a lot more power through her keyblade. She thanks Xehanort for his help, and he kisses her for the first time. This marks the official start of their romantic relationship.
• Ymir tries to tell her parents about the relationship but they rebuff her, ordering her to break it off as they don't like Xehanort. She agrees, wanting her parents' approval, and tries to start distancing herself from Xehanort, but finds herself unable to do so. Being apart from him is incredibly painful for her, so they begin to meet up in secret instead.
• Ymir's parents learn about the secret meetups and demand Ymir breaks things off with Xehanort immediately. Ymir refuses, proclaiming that she loves him, and that love is more important to her than anything. This unfortunately culminates in her estrangement from her parents.
• While Ymir pretends to be alright with this breakdown of her relationship with her parents, Xehanort admits that he feels guilty about causing it. Ymir reassures him that it isn't his fault, but he does make her promise to try to patch things up with her parents eventually, which she agrees to.
• Initially Ymir is shy about being overly affectionate with Xehanort in public, but after a several months of "officially" dating, they are known to hold hands and occasionally share a kiss in public.

During the Main Storyline

• Ymir misses the confrontation with Baldr as she's stuck in the infirmary, and she feels so guilty about it that she can't bring herself to speak to Xehanort until he, Ymir, and Eraqus happen to wind up at the cemetary together a month later.
• Although this break has caused a bit of strain on their relationship, they two of them bounce back fairly quickly and their relationship more or less returns to normal.
• Ymir does try to question Xehanort about what happened with Baldr, but he's never very open to talk about it. She ends up hearing secondhand what happened from Eraqus.

After the Main Storyline / Mark of Mastery

• During their world-exploring to prep for the Mark of Mastery Exams, Ymir visits a world called Slumbering Meadows. When the three have all returned to Scala ad Caelum, she tells the boys about it and expresses her intent to move there once she becomes a Master.
• Xehanort asks Ymir to take him to Slumbering Meadows, and they make several trips there, where they generally sit on a large hill that overlooks a small village.
• About a year later (when they're 18), Xehanort takes Ymir to Slumbering Meadows for a final time, where asks her to marry him once they both become Masters. In lieu of a ring, he gives her a butterfly pendant as a promise to always be with her. This is also when they first consummate their relationship.
• Their relationship is very strong after this, but after about another year, Xehanort starts to get restless and decides to leave Scala ad Caelum to continue exploring other worlds before their exams. He promises Ymir that he'll come back, but they start to spend less and less time together, as he isn't there.
• During this time, Ymir and Eraqus start to get closer, since they've been friends since childhood. By this point, Eraqus has devloped romantic feeling for Ymir, but won't admit them to her since he doesn't want to cause conflict. Ymir is completely oblivious to this, but when Xehanort is actually around, he notices that Eraqus' behaviour around Ymir has changed and starts to get jealous.
• At this point, Xehanort starts to get a bit more possessive of Ymir, and while it doesn't bother her at first, she starts to get a bit concerned after a while. She eventually confronts him about it, and he apologizes, stating that he's afraid of losing her. She assures him that won't happen.
• While "officially" their relationship is still fine at this point, it is strained under the surface. Xehanort is still jealous of Ymir's friendship with Eraqus, and is also spending a lot of him time researching Kingdom Hearts and the keyblade war, so they don't end up seeing that much of each other, which causes Ymir to feel as through they're drifting apart.
• During the fight between Xehanort and Eraqus over Xehanort's plan, Ymir intervenes when she discovers what's happening. When Xehanort goes to leave, she follows him, hoping to talk him down from whatever he's planning to do
• She catches up to him and demands to know what happened. Xehanort tells her to leave it alone, but she presses him until he explains. When she tries to insist that his idea is insane, he tells her to back off and leave him alone.
• Ymir refuses, and in a moment of rage, Xehanort strikes her down, almost killing her. In a panic, he takes her to Slumbering Meadows where he just holds her as he tries to figure out what to do.
• Realizing that Ymir is, in fact, still alive, Xehanort decides to leave her there, hoping someone from the village will come pick her up.
• After a few days, Xehanort can no longer deal with his guilt over leaving Ymir near-dead, and goes back to the world to find her. Unfortunately, when he gets there, she's no longer where he left her. He considers going to look for her, but can't bring himself to do it as he assumes Ymir doesn't want to see him.
• Xehanort eventually returns to Dwarf Woodlands and consults the magic mirror about whether he'll ever see Ymir again. The mirror tells him that they'll meet again on the anniversary of their parting in the place he left her.
• Xehanort starts to visit the place in Slumbering Meadows where he had left Ymir on a yearly basis. For decades, she's never there, but he continues to go every year without fail. One year, someone who looks just like her is there.
• Xehanort approaches this girl, assuming she must be Ymir's granddaughter, but then noticed the butterfly charm that he had given her is around her neck.
• He questions where she got the charm, and Ymir replies that she doesn't remember, but she thinks it was given to her "by a dear friend". She also reveals that she doesn't remember her real name, but she goes by Memoria.
• They end up talking, and it becomes clear to Xehanort that this is Ymir, but she's lost all her memories, and has somehow not aged in over 60 years
• Xehanort briefly debates just leaving her in Slumbering Meadows, as she obviusly doesn't remember him or anything else, but he ultimately decides to bring her along as he can't stand the thought of losing her again.
• He ends up taking her Destiny Islands, as he dosen't want her to meet Eraqus because that would raise too many questions.
• Ymir ends up helping Xehanort, as he promises to help her regain her memories. When she learns his plan, she has resevations, but she has nowhere else to go, so she continues to go along with it.

Kingdom Hearts III

• During Kingdom Hearts III, young Xehanort appears to Mirai several times, trying to trigger the return of her memories. He calls her Ymir and baits her by saying things like, "don't you remember me?" and, "I'm hurt, we used to be so close". He even goes as far as kissing her on one occasion.
• Xehanort's other motivation for bothering Mirai is to try to recruit her, as he thinks he can use her as a backup for one of the seven lights.
• Mirai starts to regain her memories in small chunks, but still refuses to join Xehanort. During the initial fight in the Keyblade Graveyard, a flood of memories return to her all at once, causing her to become catatonic and leaving her vulnerable to be swallowed by the heartless swarm.
• Mirai faces off against young Xehanort in the maze, where he continues to bait her. She tells him that she isn't Ymir anymore, and she could never condone what he did.
• In his dying moments, Master Xehanort does apologize to Mirai for everything he put her through, and tells her that he never stopped loving her. She forgives him and apologizes that things couldn't have been different, but says that she's happy where she is now. He congratulates her on her happy ending.