Other than the last few sentences, this was a pretty good first chapter. Though, my problem with it is that Ram wasn't given really any spotlight on how she feels. Also, I felt like the letter was much too simple. Here is a suggested edit.

'My dearest Natsuki Subaru,

Despite us only knowing for a few months, I felt I've been through thick and thin with you.'

'No one really says thick and thin anymore.'

'Ever since I have met you, you've helped me. Despite all the things I have done to you, like leaving you behind in the captial, you've never given up.'

I imagined Emilia-tan chuckling softly as she wrote that.

'Despite your flaws, and despite mine, you've cared for me beyond how many people have. While everyone was telling me, "Will you apologize for being born?', you stepped up for me, saying that I wasn't someone to be reckoned with.'

'No one says reckoned with either.' I lightly chuckled before realizing what she had wrote.

'I didn't do it for you, Emilia-tan.' My throat tightened with guilt. 'I was selfish, and I embarrassed you in front of everyone.'

'Even if you did embarrass yourself, it's the thought that counts.'

A small smile peaked through the layers of fake happiness I had buried myself into.

'Though, ever since that moment, I've noticed the small details about you. How you always try to smile, even when your heart is heavy, how you always have my intentions at heart, and despite all your flaws, you are kind, and would do anything for me.'

'Yeah, Emilia-tan. I loved you for the longest time. I never wanted to act weak to you.' My face became grim.

'Also, whenever you make a mistake or laugh or smile, I can't help but laughing alongside you. Your energy is infectious.'

Rem was looking over my shoulder, but I didn't notice that until later.

'I've been meaning to tell this to you since the time you were lying down on my lap after you had saved my life again.'

What she said next made me drop the letter.

"I love you, Subaru-kun."

My mind was swirling with thoughts and ideas.

'What? She... Loves... Me?"

I picked up the letter, finishing the last few sentences written.

'Sorry for taking so long to say that. But, it's true. I do love you. I love how you constantly strive to be the best version of yourself, I love how you don't have an evil bone in your body, I love how you snore when you slept in my lap.'

'I love everything about you.'

'Love,

Emilia'

Interesting... Pace went pretty fast at the end though...

*Smacks Lips* NOICE

woooaaah well aright

Oh, oh my... *blushes*

I thought Ram love Roswall? You lying cunt...