I'll do the OC thing depending if you want it to be male or female.

But basically she/he's been your best friend for as long as you can remember and you know that they have a weak heart so they can't do most things.

But recently, her/his heart has gotten weaker and isn't expected to live much longer, you know if their heart starts to fail you just give her/him CPR and he/she will be ok. So you have decided to make the most if your time left with them and he/she's even agreed to do dangerous things to make the most out of life.