You'd never think a game primarily focused on fanservice would be this, but the little cutscenes between Kasumi and Ayane in Dead or Alive Xtreme 2 actually work. Knowing how much of a crappy past both have had and their complicated relationship that's primarily consisted of Ayane spending the past several years hating her half-sister, seeing the two together not in the middle of yet another battle but instead just casually hanging out and enjoying each other's company is surprisingly sweet.
At the beginning of their respective story paths, Ayane merely blocked Kasumi from interfering with their plans instead of trying to kill her. At the final ending, Kasumi tried to rush into the the exploding DOATEC building to try to save Helena but Ayane stops her, saying it's too late for her to even try.
At the end of the 5th installment, both of them fought side by side at the end. When Kasumi returned the kunai that Ayane had thrown earlier, both girls smiled. Even before that, Ayane was visibly shaken when both she and Hayate killed the clone of Kasumi.
In 6, when Ayane is captured by M.I.S.T., she sends out two distress signals: One to Hayate, the other to Kasumi.
Tina and Bass posing together for the audience and just having a nice father-daughter moment even though she just lost to him in 5's story mode.
Some of the specific opening, ending and combo attack animations between two fighters in Tag Team mode could count as this, especially if the fighters never actually know each other within the game's story.
It can be heartwarming for Virtua Fighter fans to see Jacky and Sarah fighting alongside each other when you consider all that they've been through and their pseudo-rivalry in their home series.
Early in Diego's chapters in the Story Mode for 6, it's nice to see some neighborhood kids showing a ton of respect for both him and Mila, and how much they look up to and cheer for him.
Marie Rose and Honoka developing a fast friendship in 6 after Marie is tasked with becoming Honoka's bodyguard, even teaching her how to fight and training together.
Mila finally getting to meet and fight with Bass face-to-face in the Story Mode for 6. He's also a great sport about it when she wins.
The Defeat Means Friendship moment between Brad and Nyotengu when they share some ancient tengu wine that both of them had been seeking and that she stole from under Brad's nose when he found it, causing them to fight.