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."
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.
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.
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.