Each player may have (at most) one Friend card at a time. If you have a Friend card, it stays face up in front of you and provides a special benefit that repeats every round as long as you still have it. Each Friend card tells you when it activates and what it does.
It is up to you to remember to activate your Friend at the appropriate moment.
Whenever multiple Friend and/or Fable cards would activate at the same time, resolve them in turn order.