So we have three contenders for "leaving to stay with 10-C for a while and never to be seen again," - Rillek, because she's a diplomat (and her arc of recognizing the awesomness of Captain Burnham is complete); Book, because he can do the inter-species communication thingie (and because there is no realistic way back imho), and T'Rina, who is also qualified in the field of diplomacy as well as probably telepathic (but her arc with Saru is not complete; usually writers wait until they're happy together before they kill one of them off). They'd make a good team. I like all of them and hope that Rilek and T'Rina will return safely. (For Book, I just don't want a situation where he gets a mild reprimand and then everythng is fine.)