Marisa's recruitment on Eirika's route certainly qualifies, especially considering her Establishing Character Moment was her coolly asking her employer to just point her at the people she needed to kill.
Gerik: Is that you, Marisa?
Gerik: What a mess... Looks like the guild bungled. They're usually diligent about making sure this sort of thing doesn't happen. But here we are, two people of the same troop, working for opposite sides.
Natasha: On what? If this is another of your little jokes, I—
Joshua: Will I make you happy? That's what we'll bet on. Are you in, or what?
Joshua: That's everything I've got. I can't up the ante any more. So how about it? Are you in?
Also, their ending: "When Joshua returned to Jehanna, he took Natasha as his bride and claimed the throne as its rightful heir. Natasha was the perfect companion, and together, they carried the mighty desert nation to great glory."
Neimi and Colm's support, and most other supports with Neimi. They all make a point of explaining how Neimi has grown stronger and more dependable, and the implication in every single one is that she did it out of admiration for her grandfather and her love for Colm.
Forde and Franz' supports. Franz is known for his light-heartedness and admiration of Seth, so Forde is certainly surprised when Franz tells him HE is his role model and not Seth, despite Forde's insistence that there are better role models. It ends with the two of them vowing to rebuild their lives when the war is over.
In Syrene and Moulder's B support, they discuss how the two of them are often sought for guidance by the younger soldiers of the military. That's not the heartwarming part; Syrene brings up that there has been discussion of Moulder recently, and bids Moulder to wait a few days before she explains what's been said about him. Come their A support...and it turns out that almost every female soldier in the military (as Father Moulder himself states that men turn to him for spiritual support more than women, so he doesn't interact with women as much) were aware of and wanted to wish Moulder happy birthday. Moulder had forgotten his birthday due to the rigors of war, and is surprised and honored to hear so many people care for him, chiefly Syrene. Just goes to show that little miracles like birthdays can go a long way to lift spirits.
Amelia and Duessel's A support reveals that Amelia's mother is alive and well. Sometime after she was abducted, Duessel and several other knights took down the bandits and rescued her. She was in bad shape and had lost her memories, so he took her to a peaceful village where she recovered and remembered she had a daughter. When he tells Amelia, both of them dissolve in tears of joy and their shared ending has her becoming his most trusted right-hand.
Ephraim saying that Eirika can handle her own in combat in response to Seth telling her to go back to safety is pretty heartwarming.
Their relationship in general counts as this since it's built on their love and trust for each other.
Eirika and Seth's A support, where Eirika confesses that when they left their Doomed Hometown, she briefly thought they could've run away, yet repressed to say it because she knew they couldn't... and before leaving, Seth admits that he thought the same as he held Eirika during their escape.
He further states that he couldn't bear the thought as their different stations in life meant the relationship should be impossible. He states that they should just forget all about it. In their ending, he throws out his reservations and he marries her anyways. Instead of them being outcast for jumping the social hierarchy, the people of Renais rejoice and see it as a beautiful, romantic tale.
Ross and Garcia's A support. Even after Ross called him out of retirement, Garcia remains a father first and a fighter second. Also, their C support, where they talk about Ross's deceased mother.
Forde and Vanessa's A support, specifically this quote: When you first saw me sleeping, I was dreaming of running across a field. I felt this warm energy engulfing me, blowing across my face like a summer wind. It must have been your energy that made me feel that way.
Franz and Amelia's A Support
Amelia: Let me be your shield to protect you... Franz: And I will be your sword and fight for you. From now on. You're not alone anymore, Amelia. Amelia: Thank you Franz.
And then she runs into his arms and they hug. Fluff overdose, here.
Myrrh is characterized as a humble and very lonely girl. At first she seems to prefer the solitude, but support conversations make it clear that she actually longs to have companions around her who she can be herself with. This wish is eventually granted in her paired ending with Saleh, who invites her to come live in his home village of Caer Pelyn. Instead of living on her own in Darkling Woods, she gets to live in the company of friendly people who, in time, learn to not treat her as the Great Dragon, but as a normal girl, bringing her great happiness.
Artur and Lute's supports as a whole are heartwarming, but their A support is particularly so: after Artur confesses his love for her, Lute goes into a bit of a panic as she tries to recall if her books said anything about love. Artur then tells her that love doesn't need books, and that she should just be herself and listen to her feelings. After taking a moment to do so, Lute finally responds: "I'm glad... you're here."
In one support, Ross gives Amelia a necklace that belonged to his late mother.
The story Knoll tells Natasha about Lyon's first prediction.
Perhaps we were bitter that we receive no credit for aiding Grado. That we receive no accolades for our research, for the benefit it brings. I do not know. But we pressed on, nonetheless. The misconceptions linking the ancient magics to "evil" die hard and slow. Prince Lyon sought to change that. He wanted, as we all wanted, to prove that our magic had good uses. [...] With our aid, Prince Lyon predicted a great storm rising in the south. He barred ships from leaving port, and oh, how the people complained. But the storm came, and the seas raged. We saved countless lives. And Prince Lyon cared not at all if the people loved him for it. He only wanted them to be safe.
The explanation of why Innes hates Ephraim is surprisingly sweet. It's not just about Innes being highly competitive and slightly unsocialable, it's also because he sees how close Eirika and Ephraim are and he's jealous of Ephraim. He's so jealous, in fact, that in his A Support with Eirika, instead of just offering to marry her, he decides he's going to challenge Ephraim to a duel to win her heart. Now, mind you, while Eirika and Ephraim's relationship seems a little uncomfortably close, they never actually have an incestuous relationship in any of the endings. The whole thing just makes Innes look slightly irrational, silly, and adorkable. Eirika, of course, marries the guy in that ending.