To my best friend's boyfriend,
Hi! You already know me, so I don't think you want (or need) an introduction. I could start here with the usual and cringy "Break her heart and I'll break your face" line, but I won't for both of our sakes. My best friend is incredibly important to me and I think you know that by now. You've been around for a little while now, but here are some things you need to know (if you haven't figured them out already).
She can be emotional.
And that's okay, but listen to her when she's upset. She needs someone there to listen and you pretty much signed up for that position when you two became official. She'll sometimes like to bottle things up and act as if nothing is wrong, but you'll know when something is wrong. I'm not asking you to always give her advice, but instead to just listen to what's on her mind.
She can be very indecisive.
... Especially when it comes to where she wants to eat, but I bet you know that already. You've probably always made the decision on where you two wanted to eat just like I had to. And don't bother asking her what she wants to do when you're hanging out because the answer usually starts with a shrug followed by, "I don't know".
She sings... a lot.
Whether we're in the car and she hears a song come on the radio or we're walking around the mall and she recognizes a song playing in the store, expect her to stop whatever she's doing and start singing, loud enough so everyone can hear. The singing is often followed by interesting dance moves.
I hope by now, you already know everything I've listed above. You're going to find out new things about her that you never knew before. I sincerely hope you treat her like the amazing woman she is. Her love for you is unconditional and I hope it's the same vice-versa. Treat her like she deserves how to be treated. She waited a very long time for someone like you to come around, so don't take her love for granted.
Sincerely,
Her best friend who she has been through all of the cringy middle school and high school years with.