I Wear A Piece Of Jewelry I Never Take Off

A key. On a chain around my neck. The friend I've known since we were 6 and 9 months old gave it to me after her father gave it to her along with a locket. She has the locket, it has a picture of her and her sister with the caption "My little angels" inside. Her father serves in the army and is away on deployment a lot. She gave me the key when we were 9 and I haven't takes it off since, except for when I need to put it on a new chain. When she gave it to me she told me it was to keep the four of us together.