Arthur Morgan. Well, let's just say neither of you got along. Always arguing, bickering over something, trying to make everything a competition. Dutch and the others often had to intervene or it could have gotten very messy.
Dutch had sent you two on a mission and that mission included someone very special to Arthur, Mary Linton. It was already so goddamn awkward enough just listening to him try to talk to her nevermind actually do her a favor.
They used to be lovers but her father didn't like him so it never happened, now after all that in the past she was asking for help. Her little brother Jamie had joined a 'cult' as she calls it and you both had to go and find him.
You were both at the train station sending both Jamie and Mary off, you could see how Arthur looked at her and it was clear he still cared for her. Jamie thanked us both and got on as Arthur handed Mary her bag. He looked at her like he wanted her to stay but she was already leaving. Before she left she turned around and looked at him.
"Oh, you will never change." she says as she looks at him right before she gets on the train.
You turned to look at Arthur and you chuckled slightly, not understanding how much that really hurt him.