Character switching and the relationship
Two leads instead of three, and an interface that puts a heart between them rather than a skill bar.
Grand Theft Auto V let you rotate between three men who mostly tolerated each other. Grand Theft Auto 6 has two leads who are together, and the switching interface reflects it: the character wheel shown in the footage puts a heart directly between Jason and Lucia.
That is a small piece of interface doing a lot of signalling. In GTA V the switch wheel was a utility. You picked whichever character was closest to the mission or had the right special ability. A heart in the middle says the relationship is a tracked thing rather than a story fact you are told about in cutscenes.
What is reported to feed it
Texting, dates and time spent together are all reported to shift where the two stand with one another. None of that is a Rockstar statement, so treat the specifics as reporting rather than confirmation. What is on screen is the heart and the two portraits.
Why two rather than three
Three protagonists gave GTA V variety at the cost of intimacy. Any one of Michael, Franklin or Trevor could be parked for hours. With two, both are always close to the story, and a partner mechanic becomes possible in a way it was not before. The partner is also a tracked item in the evidence system, which means the other character is a liability as well as an asset.
What is not known
Whether switching is free-roam at will, restricted during missions, or gated by story position has not been shown in full. GTA V allowed switching almost anywhere outside missions and used it structurally inside them. Rockstar has not said whether the relationship can genuinely deteriorate, or how far, and nobody outside the studio knows what the heart is measuring.
Related: Criminal Profile.