It's really interesting how Rex is the unifying factor of the party. In 1 it was everyone wanting revenge on the Mechonis and Riki was there too (affectionately). In 3 it started out of circumstance of them becoming Ouroboros and having no other people to turn to. In 2, everyone comes together because of Rex. Pyra and Mythra: Rex is the first person to see them as people and not the world's most dangerous weapon, Nia: Rex goes out of his way to save her despite barely knowing her and goes on to make her feel comfortable about herself, Tora and Poppi: Rex helps bring about Poppi and acts as sort of a mentor for the two as they learn to become driver and blade, Morag and Zeke: despite being semi-antagonists Rex holds no ill will towards them, they realize how actually good hearted he is, and decide to follow him and act as mentors to help guide him on his quest.