Flaccid 20 Fluffer

In your time around adventurers, you have developed an almost sixth sense to when your allies need a little encouragement.  As a reaction, when an ally makes any d20 roll, and with the relevant modifiers the total equals 20, you can change this to a natural 20.  You can use this a number of times equal to your Charisma modifier.  Any expended uses are restored after a long rest.