Being best friends with Merida would include:
(For anon)
Things being turned into friendly competition. “First one to that tree gets the other’s dessert.” “Bet I could climb higher than you.” And so on.
Merida venting to you when she gets frustrated. She’d just need you there to listen to her so she felt heard.
Getting into trouble together. Her parents would follow the rule that while you’re under their roof, you get punished just the same. Well, at least the Queen would,
Merida helping you learn to shoot a bow, or help you improve if you already know the basics.
Going on adventures together; riding further than you’ve ever gone, going to places you’ve never seen, and finding your way back again. As long as someone knows where you are.
Staying up late, laughing the night away while you tell stories and old fables.
Pulling pranks on each other now and again. Nothing that purposefully causes damage, of course. Just innocent fun.
Merida making sure that you knew, no matter what, you were always home when the two of you were together.