Somethings seemed off so I tracked down the raws and compared that with the translations. There are a couple of mistakes, but one thing that's pretty important is that in the first chapter right after Soara got kicked out of the party and the Hero party was trash talking her, Arnold's line is something more like:
"I don't remember that woman who's only strong enough to cut a boulder being of help to us."
So both Arnold and Riruka seemed to be judging Soara on what they excelled at, so from what has been revealed so far up to the raws, the party lineup was something like this:
Zenon - Main Physical Attacker
Riruka - Healer (Maybe main magical attacker, but there's no confirmation that she has offensive spells so far)
Arnold - Main Physical Attacker
Soara - Secondary Physical Attacker, Secondary Healer, Secondary (main?) Magical Attacker, Support (Buffs + Barriers, assuming the barrier in chapter 1 was made by Soara)
Soara tried to compensate for her lack of a S-Rank skill by basically being a jack-of-all trades and I think the idea is that although Riruka is better at burst healing, Soara could heal more often because she uses lower-level spells and her support spells made it so that they didn't need as much healing in the first place.
I've heard of a saying in DnD that goes something like:
"A good party does not need a bard but a good bard does not need a party."
In this case, Soara was the bard that filled a bunch of holes in the party.
Funny thing is, there isn't even a 4-person party limit so they could have just got Eris to join them without kicking out Soara and everything would have gone smoothly, but Zenon was too obsessed with having a full party of S-Rank skill users.