A bond between soldiers, like in Firaxis's Xcom 2. Two soldiers become friends, and get their stats boosted (probably in form of comendation). Possibly even several types of bonds - simple friendship, friends with benefits, lovers, married. If one of the agents die, the other gets a malus to stats (probably just temporal).
Other type of bonds - 'Dislike' and 'Hatred'. If these guys on the same base they get malus to stats.
Whaddaya think?