Evidence of something real
Regulus reads the letter over and over. He reads so many times until he can recite it. So those words could soothe him in his mind during those bad days when nothing not even breathing makes sense anymore. It’s the only light he has right now.
Regulus always felt alone coming back to his home ever since Sirius left. He accepted his fate years ago. But then James Potter happened and suddenly there was someone who cared for him. Who saw him more than the heir of The Noble and Most Ancient House of Black. Who didn’t think he was a lost cause. Who even claimed he loved him.
This summer was worse than the previous ones. His bed feels lonelier now that he knows how it is to fall asleep next to someone you love. His skin is starved without the touch. His eyes tired without looking at that smile that sets fire inside him. His mouth is without motivation when no one is listening to him. Everything feels cold. And it makes sense since there isn’t Regulus’ sun to warm him up.
The only thing he has is those letters. And he clings to the words because they are proof that someone is waiting for him. That all of this wasn’t just part of Regulus’ imagination. Because it’s easy to forget in this house. With each day the memories feel more distant, less focused. Like a dream. Like Regulus made it up. His brain is constantly telling him that of course, he made it up. Why would someone care about you? You are not worthy of anyone's love. his mother's voice echoes in his mind. And when Regulus feels like he is losing his mind, a letter will come to remind him to hang on.
They can’t text often. Even this rarely is risky. If his parents found out, in the best scenario, they would send him to study in France, in the worst… They can’t even send letters directly to each other. They use Evan, so Regulus can pretend those letters are from him. His parents believe his lie so far, but Regulus can’t keep those letters.
The words are too dangerous. So, he reads them over and over, until he has it memorised. And then he burns them. The only evidence of his relationship. He watches them give up under the merciless flames praying that his mind won’t betray him. That he can hang on for another month until he can see his sun. Until he can touch him and be touched so his mind would finally believe him that it’s real.