For ever 10+ Damage the other character does, this character would gain 1 Dualism Stack.
Using Dualism doesn't require an action but is rather a bonus action.

Dualism Stack Uses:
1 Stack = +5 HP to a person he touches, or the other character (Must be within 50 feet).
1 Stack = +10 feet movement speed to the next person he touches, or the other character (Must be within 75 feet).
1 Stack = The ability to cast a damaging cantrip.
1 Stack = Add a +1 Damage on the other character's next attack (Must be within 50 feet).
2 Stacks = Cast Enlarge/Reduce on the next person you touch, or the other character (Must be within 75 feet).
2 Stacks = The ability to cast "Dash" (Dash to any direction by 35 feet or less).
3 Stacks = The ability to cast invisibility on the next person you touch, or the other character (Must be within 50 feet).
4 Stacks = Give someone a +2 on Armor Class for 2 Rounds.
5 Stacks = The ability to teleport the other character to you (Must be within 100 feet).
5 Stacks = Swap positions with anyone on the field, if it's an opponent roll a Dexterity Saving throw to cancel the spell (If it fails you don't lose the stacks).
7 Stacks = The ability to Create a 1 tile barrier over the other character (Must be within 50 feet).
10 Stacks = The ability to give any user X2 actions for 1 turn.
14 Stacks = The ability to make the other character use your own HP bar instead of theirs.
20 Stacks = The ability to make the next action the other character does succeed, and if it's an attack, a critical hit.
30 Stacks = Give a character a +2 Modifier on 1 stat for 1 day.
75 Stacks = Learn the ability to keep your stacks
200 Stacks = Become a LVL20 Wizard for 30 Seconds

Dualism Requirement:
Must be Elf or Half-Elf.
Must be Blood Related or Married with the other user of the feat.
Cannot be the same class.
Cannot be used if the other is 150 feet away.
Cannot gain stacks from attacking a passive or inanimate object.
Cannot gain stacks from someone allowing you to hit them knowingly or unknowingly.