Out loud, Arthur had always said that Merlin was a coward.
He had teased him mercilessly, and Merlin, being Merlin, had always taken it in good grace, sometimes even laughing along with Arthur and the other knights at his expense.
But as Arthur and his knights rushed headlong into yet another dark battle, Arthur suddenly realised that there was Merlin.
Always by his side, never even contemplating that he didn't have to be there- that he actually shouldn't be there, and yet Arthur was grateful that Merlin wouldn't have it any other way.
He wanted his friend by his side.
Later on, when the battle was done, the victory was had, and the injuries healed, Arthur took the opportunity as he sat eating his dinner and Merlin tidied his room before the King slept, to remember all those times when Merlin was right there.
Never a coward.
He smiled fondly, not even minding when Merlin looked up and saw the King of Camelot giving a servant such a brotherly smile.
Merlin smiled back.
And instead of teasing Merlin for the soppy smile and calling him a girl, he let it slip in his own smile that he valued Merlin. That he wanted him serving by his side always as his friend.
And instead of teasing Arthur and calling him a spoiled prat, Merlin smiled to let his friend know that he would always serve his King as his friend back.