PC Gamer highlights a new mod doing the rounds for XCOM 2 that adds relationship buffs for your soldiers. The mod is titled RealityMachina's Squad Cohesion.
More information.According to the mod's blurb, "leveraging a 'squadmate score' system left in by Firaxis" allows Squad Cohesion to "give small buffs for squads that trust in each other." In practice, this grants buffs (and debuffs) to squad members who forge strong fighting relationships by way of cohesion points. For example, saving vulnerable comrades from seemingly inevitable death, healing them, or dragging their unconscious asses to extraction points, enables soldiers to accumulate points and thus makes those particular partnerships stronger.
But before you get all mushy on me, let's take a moment to consider the pain felt in the aftermath of mismanaged tactics. You know, those operations that result in the death one of your best soldiers. Horrible, isn't it? Now imagine how your digital squad feels. You've forced these relationships upon them, and your tactical inadequacy has directly resulted in the death of their BFF.