Heart Piece - (n) more commonly known for their use in the Zelda game series, a heat piece is someone in a person’s life who just after mere moments becomes a part of said person’s life forever. There is a connection, a bond between the two people involved that can only be broken by severe injury or death.
"When did you realize He was your Heart Piece?"
"From the momet I met him, we clicked on so many levels and we can put up with each other."
"Isn't that like a best friend?"
"Sort of, but Heart Pieces are forever."