It's dangerous to go alone! Take this! Well, in this case, "this" happens to be the Pair Up feature ! Liberal and strategic use of pairing up in its two flavors will allow you success and survival during the war campaign. There are two flavors of pairing up: Adjacent Pairing and Defensive Pairing .
Adjacent Pairing requires the two characters to be standing in separate tiles, adjacent to each other. Adjacent diagonally will not work! This allows both characters to freely attack, but does not boost either character’s stats aside from Hit, Crit, and Dodge. Guarding doesn’t happen as often in this pairing. This pairing is more offensive. It allows you to use both characters to attack, so you have two attackers instead of one, which can ease the pressure off of your army if facing a particularly large or dense army. This also allows units to "go twice" in one turn, so to speak. Enemies like to use this a lot.
Defensive Pairing places two characters together in one tile. The character in the defensive stance will not ever attack. However, in addition to Hit, Crit, and Dodge boosts, the attack stance character (the one who will be attacking enemies) will gain flat boosts to their stats depending on the character that is in defensive stance. The boosts depend on the character and the support rank with the character. Married pairs will enjoy bigger boosts than incompatible characters.
However, you lose the attack power of one unit, and if everyone in your army does defensive pairing, you have halved the number of units you have that can attack, so keep that in mind. Units in defensive stance also don’t gain much experience since they can’t attack.
Whether you go adjacent or go defensive, unless you have a really good reason, you should never allow units to rampage off alone.