I last watched “Serenity”
I think pretty much any of the characters would be awesome companions! I think it would be hysterical to see the Doctor try to stop Jayne from killing things, or see him react to River’s nonsense when she’s not so coherent. If River were a companion, I think Simon would have to be with her, and he’s such a proper uptight Doctor I think the dynamic between the three of them would be…. FANTASTIC.