itch.io

It's a shamrock, in the prohibition when alcohol was illegal people would use pins among other things to show that they are wanting alcohol at bootleg bars and so on. So I'm guessing that's the significance of the pin

In Mark's route, Grey touched the clover pin at the mention of his grandmother when having coffee with Simone. Maybe it was a gift from her, and also a clear symbol of his Irish heritage. In French, clubs is trèfles, which means clover.