You can assign a second unit to your selected main unit
to increase your HP and ATK, and combine skills to battle.
Go to Edit Party to choose your main unit and armament,
then select a second unit to be assigned from the small unit window at the bottom of the armament selection screen.
Go to the Mana Board of the second unit to check your abilities
and see whether they can be applied when assigning seconds.
You can also check the details of a unit from the unit list on top of the Units menu.
Select the skill [Preview] at the bottom of the Unit Details to watch a unit's skill animation,
compare each unit's skills and abilities, and assign seconds strategically.