tl:dr - Have your units become friends by fighting 1000 times together before tackling hard content or being on the same PvP team. Once in battle, stay close to one to three of your friends.

What is affinity?

A bond of camaraderie that is gained by fighting alongside your teammates, that boosts unit performance when they are near each other. At max rank it provides a boost of 10% Agi, 15% Dex, and 10% Luck to units that are within 2 spaces of each other. It partially stacks, so units in a tight-knit formation leverage this the best, more than a scattered group of lone wolves -- but having at least one nearby ally is always a good idea.

Note -- these bonuses to Agi, Dex, and Luck are based on the unit's base stats only! Other improvements to their stats, whether by bonus from their ability board, equipped esper, gear, VC, etc, provide no additional bonuses via affinity. Which, in practice, means your primary boost is closer to 6% Agi, 11% Dex, and 6% Luck. But welcome perks regardless!

These bonuses to agi, dex, and luck will result in your unit doing slightly more damage* while having more rapid turns (double turning, or preventing a double turn!), along with an increase to accuracy, crit chance, crit avoidance, and dodge. So if you are struggling against a high evasion target, an action you might do is being proactive with your support units and also bringing them forward, so your damage dealer can leverage affinity to the benefit of all.

*For abilities from classes that draw damage from agi/dex/luck in the first place. Knight, Soldier, Samurai, Monk, and Viking skills do not draw damage from agi/dex/luck and opt to get all their attack purely from the ATK stat. Those units are more indifferent to the POWER OF LOVE AND FRIENDSHIP.

The Perks

^{The main hump is between 3 to 4, oddly enough.}

Diminishing Gains

Like I said, affinity partially stacks. Having at least nearby one ally nearby is great. Having a 5th ally nearby does literally nothing. The following is a crew at 5/5 affinity. You're capped at 20/20/20.

If an ally is KO'd, their decomposing corpse does not contribute to affinity bonuses.

Left: Pitiful (baseline) Murmur. Howl alone! Howl Alone! Right: Murmur surrounded by friends and loved ones

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A more practical example, with gear/esper/VCs in the mix. Left is solo, right is full affinity

The Perks of 20/20/20

Using Ramza and Shadowlynx as guinea pigs with decent gear/esper/VCs, the following examples are the outcome when both units are not supported by friendly units, when only Ramza is supported, when only Lynx is, and finally when both are.

Fully supported, Ramza's damage is increased by 1.6%, and his accuracy increased by 17%. Also, his critical rate increased by 8.1%.

Fully supported, Lynx's evasion increased by 18%, also her chance to be crit was reduced by 6.5%.

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Left - Solo Ramza, Solo Lynx. Right - Supported Ramza, Solo Lynx

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Left - Solo Ramza, Supported Lynx. Right - Supported Ramza, Supported Lynx

Gaining Affinity

Being in a battle together = 5

Using a skip ticket = 0

Ending a turn directly adjacent to the unit during that battle = 5 [Only once per battle]

Being KO'd at fight's end = Negates all affinity gain for that battle.

Since you need 5000 total exp to reach 5/5 affinity, this works out to somewhere between 500 and 1000 battles together, assuming no KO's.