Two mercenaries who deploy together 3 times form a relationship. Which one they form depends on the gap between their ages and their skill. It shows on the Record tab of the mercenary sheet.
Comrade
Two people whose age and skill sit near each other. When they stand in the same row, both gain 12% attack and defense. Different rows give nothing. One person with several comrades still gains it once.
Rival
Two people close enough in age and skill to measure themselves against each other. When they deploy together, both earn 15% more experience. Rows do not matter. It reads as a hard word and works as a good one: rivals pull each other up.
Mentor
Two people far apart in age and skill. When they deploy together the junior earns 25% more experience. The senior gains nothing. This is why sending a newcomer out beside a veteran raises them faster.
Feud
Two people who fell out. It grows after a lost expedition, or while morale sits under 40. While both are in the same party their attack and defense drop 8% and morale bleeds every week. Clearing a floor together can mend it.