Pairing Up is a game mechanic in Fire Emblem: Blade of Undeath. It allows two characters to move, act and fight together, also getting bonuses to fighting and to their stats. This mechanic first appeared in Fire Emblem: Awakening.
The leading character gains bonuses to his stats according to the formula:
((Support Level) + (Leader stat)) - (Partner stat)
Support Level: 1,2,3 or 4, according to C,B,A or S.
Leader stat: The Leader's stat value.
Partner stat: The Partner's stat value.
The leader gains a fixed +1 to movement when paired up with a mounted unit.
Paired up units gain the following bonuses when fighting (And also when adjacent without pairing up):
The leader gains these bonuses, according to it's support with the partner:
None: +10 Hit
C: +10 Hit, +5 Avd
B: +15 Hit, +10 Avd, +5 Crit
A: +15 Hit, +10 Avd, +10 Crit
S: +20 Hit, +15 Avd, +10 Crit, +5 Def
After the leader attacks, the partner has the following chanses to attack too:
((Support Level) + (Leader Charisma)) + (Partner speed)
Support Level: 1,25, 45 or 65, according to C,B,A or S.
Leader Charisma: The Leader's Charisma stat value.
Partner speed: The Partner's speed stat value.
The partner has the chance to Critical-Hit just as if he attacked on his own.
The partner has the following chances to defend the leader, causing the attack to miss:
(Support Level) + (Partner defense)
Support Level: 1,5, 10 or 20, according to C,B,A or S.
Partner defense: The Partner's defense stat value.