four-leaf clover
1. Literally, a clover with four leaves instead of the usual three, traditionally thought to be a good-luck charm due to its rarity. You found a four-leaf clover and you saw a shooting star? Wow, you must be really lucky today!
2. By extension, any person or thing that is considered a bringer of good fortune. The new kicker has been something of a four-leaf clover for the team, as they've won every game since he signed with them.