Healing and buffing each other can provide positive Affinity, but it's pretty random when you're out in the field and your control is limited. These relationships aren't set in stone - good relationships can still collapse while bad ones can be healed, but we find it's a lot easier for bad relationships to grow organically, especially once characters start having Meltdowns. Consequently a team of best friends is a powerhouse force, while a group of spiteful characters is very likely to fail. Two close characters might heal each other for free or add damage to their attacks, while characters at odds will trip each other up, cause stress damage with insults, or even stop them from healing each other if they don't trust them. These relationships do a lot to help or hinder you, as they effectively provoke random buffs, penalties or even behaviours during combat. You can check on these relationships in the character screens, but when characters reach a certain level either way, they gain a random positive relationship - Inseparable, Respectful, or even Amorous - or a negative one, such as being Hateful or Suspicious.
Actions taken in and out of combat can have positive or negative Affinity between two characters, making them like each other more or less. The relationships between individual characters is now a major mechanic, replacing the insanity mechanic of the previous game.