Swap HP with adjacent ally (neither unit can go above their max HP).
This skill is second only to Sing and Dance for utility, by giving you a great deal of control over your team’s HP pool. There are many reasons why one would want to do this. Firstly, it can be used as a pseudo-heal. It’s a great answer to mono-color arena defense teams: for instance, if you run a tricolor team and encounter an all-blue team, you can use Reciprocal Aid to constantly refresh the HP pool of your green unit, allowing them to KO all four foes. Secondly, it can be used to help your units meet certain HP requirements. If your unit with Quick Riposte falls below 70% HP, they can trade health with an ally to bring them back up over the threshold. Conversely, the same applies for low health thresholds: You can use it to bring a team-mate to the HP the need to activate a Defiant skill, Vantage or Desperation. The skill’s function is such that you really only need it on a single team member: If unit A falls to critical HP, unit B can refresh their HP with Reciprocal Aid. If unit A falls to critical HP once more, unit B can trade with unit C, and then unit A, effectively letting unit C and A trade health despite neither of them having the skill.
As this skill only needs to be present on a single ally, consider carefully which unit you place it on. There are limitations: Two full-health units cannot use this skill. Units can’t heal more than their max HP, and can’t trade more than their max HP. Therefore, this skill should be present on the team member with the largest max HP. It also helps if all team-mates have high max HP or very similar max HP. Skills that restore HP also synergise well, most notably Renewal and Falchions, but also Breath of Life, Aura and to a lesser extent, Sol and Aether. As mentioned before, it synergises very well with HP-threshold skills, granting increased control over their activation. Of these, Vantage and Quick Riposte are the most noteworthy, as they force enemies to take a lot of extra damage.
This can be an annoying skill to face: if a single enemy keeps getting their HP refreshed, they can wear down your counter to them and score KO’s on your team. Try to KO them within a single turn. One flaw of this skill is that it weakens a unit who would otherwise be on full HP. If this is the unit with Reciprocal Aid, try to eliminate them quickly. It’s also a good counter to itself: If the enemy keeps refreshing a single unit, you can do the same to keep your unit in the fight.