That was beautiful.
Oh my god, that was so emotional! I really was able to get into Lucy's head, and empathize with her. The suicide promise was just... wow. You've really managed to capture just how much Lucy needs Mike. I also like how you're showing that Mike really does care about Lucy as well. I like how you've captured how strongly these two kids feel for each other, and it all feels very natural and real.
Alas, there are things I must critique.
"Paulo was wrong. I don't care what he or anyone says about you! You're not a wimp, you're not a bad friend, you're not a loser, and you're not a piece of shit!" Lucy continued yelling into his chest. "You're my best friend Mike. No, you're my only friend. You're the only person who has ever really been there, the only person who could ever really help me. I don't want you to leave, I don't want you to move to Richter with Sandy, I don't want you to die! I… I don't know what I would do, Mike. If you left me… I know it's selfish… I know it's self-centered… but don't want to suffer anymore. But even if I have to suffer, even if I have to spend the rest of my life in pain, I want it to be with you. As long as I'm with you, no matter how bad life get's, it will never be this." Lucy slowly quieted her voice as she spoke. "I don't care how sick you get Mike; I'll be there for you. I don't care what everyone else does or say's, I'll be there. No matter what, no matter what, happens. And the day you finally… pass. Whether it's sooner or later, I'll go with you." Lucy said to him, tears drenching the fur beneath her eyes.
First off, don't use apostrophes to pluralize. It should be "gets" and "says" not "get's" and "say's." Next, it seems kinda off when Lucy is saying she doesn't want to suffer from Mike's suffering anymore. What does she mean will never be "this?" Never be what? Liver cancer? Third, instead of saying "No matter what, no matter what, happens," try "no matter what happens - no matter what!"
Next, I understand that you're trying to show how distressed she is by having her babble on at the mouth, but it would be wise to break up the speech a little. Show what she's doing! Show what Mike's thinking! Have him notice how soft her head his, or the wetness of her tears.
Oh and before that when you have "Mike simply stood there and listened while he consoled Lucy," is a little strange, because instead of consoling Lucy, Lucy just gives him more stuff to console. If you're going to put that there, maybe him say a few kind words?
Also, where exactly are they? School? The bathroom? Somewhere private? Somewhere public? Are they making a scene? It isn't exactly clear.
Finally, this takes place after "A Distance Apart" right?
Looking forward to the next chapter, and it's good to know I'm not the only MxL shipper. I was sure the entire fanbase hated that pairing.