You probably know the drill.
In generic game mechanics:
Draw two intertwined hearts on your character sheet. On one side, you put your character’s name. On the other side, you put the name of your character’s one true love (if you don’t like the concept of all-conquering, mystical true love in your game, just put the name of somebody who is really important for your character there – however, you still get to put only one name there, ever).
If your character dies, you may colour in or otherwise mark one of the hearts. The character immediately returns to life. (If your rules have a resource such as fate points or similar, which allows you to avoid character death, then colouring in the heart is treated like a use of that resource.)
Whenever your character’s one true love is in grave danger, the character feels it like a sudden stabbing pain in the heart.
If your character’s one true love dies, you may colour in the other heart. Your character’s one true love immediately returns to life.
Each heart may only be coloured in once.
Alternatively, your character can love the world.
You may draw three hearts for others on your character sheet, but none for your character.
If you put a name next to a heart, you are warned of great danger.
You may also choose to leave out names next to one or more of the hearts. In this case, you receive no warnings but you may use the heart in question to save from death any creature you like.
Additional variants could include a big heart to gain more hearts (paid for with some sort of resource, most likely), using the hearts for positive modifiers/automatic successes instead of escaping death, temporarily adding names for equally temporary relationships, and so on.