How do you unlock a hero skill?

Whenever you recruit a hero that you already have you will get a duplicate hero card. These can then be used to unlock the skills of the same or higher tier heroes, or you can discharge them, which turns them into hero Target Points that you use to level up the skills. If you are automatically discharging hero duplicates when you recruit them, you need to turn that off so that you can keep some A tier cards.

Each skill you unlock will need a certain amount of cards from heroes of specific tiers.