A strange presence watches over your character, keeping her safe from harm — or so it seems. She gets into and out of serious scrapes without any real harm.
When a bad situation could go either way, it always tips just right. Your character may just be lucky.
You have two options for the Guardian Angel Merit.

You might decide to specify what exactly the presence is that protects your character, give it traits and fully defined capabilities, and have the Storyteller control it during the game. The creature might be ghost or spirit (see p. 199), or a supernatural creature of some kind — perhaps even a vampire, werewolf or mage. The Appendix to this book gives some basic information on such creatures. Making the Guardian Angel into a character this way means that what it can and can’t do is very well defined, but it also means that the Angel can die. The Angel should still be invisible much of the time, only appearing and helping the character indirectly.

The second option is that the character just seems to get all the breaks. The “Guardian Angel” here is metaphorical, rather than being an actual, sentient creature. Before every chapter, the player rolls Resolve + Composure. Multiply the successes by two. The result is the number of bonus dice that can be used on any roll during that chapter. The dice can, instead, be applied to characters acting in direct opposition to the protected character as penalties. Each die can be used only once.

Example: Alice has the Guardian Angel Merit. Before the session starts, her player rolls Resolve + Composure and gets three successes. She therefore has six bonus dice for this chapter. During the session, Alice winds up running away from one of her teachers, a man who turns out to be something other than he appears. She hides from the teacher, and her player applies three of the bonus dice. The Storyteller picks up some dice to roll Wits + Composure for the stalker, and Alice’s player, not liking the size of the dice pool she’s seeing, applies the other three dice from her Guardian Angel as a penalty to the Storyteller’s roll. The roll fails, so Alice is still hidden — but she’s used up her Angel’s influence for the chapter.