A MultiVersus dataminer has detailed an affinity system in the game's files that originally let players earn characters for free through a gacha system, but now seems to have been retooled to have fighters become more friendly with you the more you interact with them.
As is to be expected considering that it's still in beta, MultiVersus has quite a few unused gameplay mechanics. One such unused mechanic is an "affinity" system. Reliable MultiVersus leaker LaisulMV detailed the system in a tweet after finding evidence of it within the game's latest update. LaisulMV said, "Early on, MultiVersus had an "affinity" system, where to unlock characters for free you had to give them gifts, like a gacha system. While all the gifts and gacha elements have seemingly been removed, the "affinity" system itself seems to be staying in some form."
LaisulMV continued, revealing that the system is still in place now, but has been retooled, "It got a few updates this patch, and its main use appears to be to greatly expand the number of lines you can get when clicking on the character, with them growing friendlier as it gets higher."
Interestingly, we can actually see bits of this system in action from datamined voice lines. Back when MultiVersus was in its closed alpha state, several dataminers went through the game's files to put together all the character interactions and voice lines. Each character had quite a lot of voice lines for when the player clicked on the character in the main menu, ranging from the character being fairly unfamiliar with the player and not saying much, to being happy to see them, to acting like you're their best friend.
This seemed to imply that fighters would get more friendly with you the more you interacted or used them, something that's not present in the final game. In fact, the final game only has a limited number of voice clips that play when you click on a character, and they all seem to be from when the character is unfamiliar with the player, leaving a ton of voice lines unused.