1. Eir, she absolutely must be included, and there's no debate about it. She's the strongest member of the group.
2. Renold is also, in my opinion, an essential member of the team. While he doesn't quite reach Eir's level, he's still extremely strong.
3. Rohei is the third essential character in the group. We can argue endlessly about his usefulness as a combat unit, but he's definitely not weak. Moreover, he's the only healer in the group, which makes him necessary.
4. Anders should be taken along, at least because Elheim is his homeland, and he should know what's what here. Besides, he's one of the few ranged characters, with paralysis and elemental damage as a nice bonus. And he has a cat.
5. Here, things get much more complicated, as there are several good candidates. Anselm's ability to inflict AoE paralysis and simply provide buffs to allies speaks in his favor. He increases the reward from quests, has a chance to upgrade his healing skill and become a backup for Rohei, and can also support the Mirai with prayers. However, he's quite fragile and not very useful as a warrior. And Aretz is a coward and is unlikely to want to go on a dangerous mission. Alessia is even more fragile, but she can act as heavy artillery, leaving only charred remains of enemies, allies, and other Rainers. Kalentia, on the other hand, is a decent and relatively strong combat unit, with access to the same paralysis and the ability, like Anselm, to support Rohei with prayers. She doesn't bring anything particularly interesting, but unlike Anselm and Alessia, there's no need to protect her. I might even get AoE paralysis on auto-attacks if I decide to take "Cyclone" instead of "Discipline".