Choose a unique Character unit card from your hand and stack it under one of your unique Characters with a different name. (Follow normal stacking rules.) Until that Character is discarded, when the Character battle step ends, put the chosen unit card on the bottom of the unit's stack. If you have another copy or version of the chosen unique unit card in play while the chosen unit is on top of the stack, you may retreat the contested unit card instead of discarding it.
Treat the new unique unit card as if it is another version of the unit while beneath the stack.
You may bring it to the top during your build step as normal, paying the difference in build cost.
The stack's name changes along with everything else while the new card is on top of the stack.
You can't remove the unit card from the stack. It is considered part of the unit.