I'll share some information, since there is a lacking of ideas.
First of all, the battle system isn't "experimental" it's a simplified form of SaGa Frontier (PSX), and I believe the older Romancing SaGa games (from Famicom) - even the attributes (WP/JP) come from those games. Characters improve by fighting stronger opponents. Each sprite is a type of enemy, and by defeating them again and again the stronger versions come up. Stronger ones give more bonuses to HP/WP, weapon proficiency, and have more chance for learning new weapon techs.
However, this game system is very simplified - spells are useless, don't bother. SaGa Frontier also had an extra attribute called LP (Life Points), which was fixed for each character, and would show how many times HP could drop down to zero before truly dying. A char with 0 hp hit by an enemy attack would take a -1 LP directly instead of HP damage - crits would take 2 at once. Since the max would be 9, that would make an unbalanced system like this one pretty hard to deal with.
Another aspect - boss battles are NOT scaled. the very first boss at the tutorial cave will always be a breeze, even if you're in your 50th playthrough, and some bosses are truly hard at the start because of it (the 3 devils for example).
about the characters:
1. Nina and Rifina are mutually exclusive - you cannot have both at the same playthrough. I *think* this is a glitch: Rifina has 3 rape scenes while imprisoned, but the first and second ones are blowjobs only. However, the game treats it as she were impregnated (which only happens at scene 3). Anyway, you must choose between them.
To get Nina, go to the cave where the kid who stole you money went in, and give them the money. She'll join and you'll get Rifina scene 1, disabling her for the playthrough.
To get Rifina, rush through the lake (where the slave auction is held), stop auction; then, once you're back in city, rush all the way to the east side (don't lose to the bandit on bridge or Rifina will get a scene; defeat him or pay up). If you save her without seeing any rape scenes she'll be a party member after your meeting wth the empire generals.
2. Dietlinde will be a party member in all but two scenarios:
- if you choose to help her against the phantom at the cave and lose the battle, THEN defeat her at the border fort; she dies;
- if you choose to help her against the phantom at the cave and lose the battle, and lose vs her at the border fort; She and Eris have a lesbo scene (can be repeated by visiting her at the fort thereafter). She won't join the team.
If you ignore her at all before saving Rifina she'll be a party member in any occasion.
3. Marguerite will join your party in two scenarios:
- If, before saving Rifina, you helped her at the forst vs the nymph, and LOST the battle. When you talk to her at the forest fort later you'll fight her, and if your corruption is somewhat mid-tier she'll join after;
- If you helped her vs the nymph, won, or ignore her, she'll join as long as you get Makina before her and have her at the party when facing Marguerite at the forest fort.
If you killed Makina before reaching her she'll never join the party.
4. Makina joins the party if, before facing her in the mines, you went to the forest fort, read both entries at the library shelf, and found the secret passage to the spirit orb. After defeating her, she'll "revive" and join you.
Another scenario is if you failed the battle helping Marguerite as stated above, and got her in your team; if she's present at the battle, she knows what to do about Makina even without the spirit orb.
In other scenarios she dies (locking Marguerite too).
5. Klaus is a mystery to me so far; he IS mutually exclusive with Rifina (by helping him time passes and she gets scene 1 anyway), but I still didn't find a way to add him to the party. I'm totally sure you MUST win the boss battle at the devil ruins when helping him - if you lose, he will state you'll be dead at any means when you fight him at the end of the game.
If you won the boss battle when helping him, his speech changes a lot when fighting him and winning on the castle, but he still dies. No idea how to get him.
6. Not the slightest idea about Neidhardt, but he's a character too. I'm sure it has to do about the Devil King before going to the castle, but as I'll explain in the next paragraph...
7. No fking idea about the Devil king. There are 3 devils in the game - they are ridicolously strong in your first playthrough, but beatable after a few iterations, even on hard:
- the first one is the church priest, where the little nun Sera can be found. There are two possible places to fight him: if you "join" the church, and buy all 3 items he offers, he'll disappear with Sera, but you can find a hidden entrance at the statue behind the altar. If you order him to shut down operations at once, he'll head to the hidden underground cemetery. If you fight him in the cemetery, you'll gain access to the church underground later, but the opposite isn't true;
- the second one is on the dreamworld you can find at the small village near the border fort. You need to pass a stupid puzzle with two maids' rooms while in the dream (if you can't locate the different one, kill all of them. One will pop without fight, you can leave the room at that point and repeat to the other room). Then go to the leftside room and, I don't know, rotate twice around each statue and leave I suppose. You'll be in a new area. It's a straight line to the devil then.
- Once you dealt with Makina, Marguerite and Dietlinde, regardless the results, a new event opens up at the cave on the far northwest (further north where you fought Makina). Kill the dragon, get the legendary herb to the girl, devils appears, kill him.
Then I'm stuck. If you helped Klaus on the devil ruins, he mentions there is no way to open the door without 3 artifacts scattered throughout the land, and I suppose the devils have something to do with them, but I still don't know how to open the door (where I believe the devil king can be accessed).
8. No fking idea about the boulder on the small village where you entered the dreamworld. Editing, found out how to open it. Pretty anticlimactic too.
When you meet Nina and the kid who stole your money at the end of the cave, ask the money back without fighting. They return it and go away (Rifina gets a rape scene and cannot become a party member due this). Then, after you rescue Rifina, go to the village and meet nina at the bottom right house for an event and a scene.
Finish a general (Dietlinde is the closest), then return to the village - there will be an event with the mayor. If you stop the scene there you'll be at the blocked cave. It's a fight with 4 rats. That's it. 4 rats.
Now, only thing left to discover is the damn door at the devil ruins, which I believe will tie up how to get Klaus and Neidhardt on party as consequence.
Oh, and a small tip for easy plays: Eris wth spear, learn a skill called Spear Shower, and equip the hand of silver you get on the dreamworld. Spear shower is a special skill which improves with spear proficiency, and the multiplier is insane. There is a version of this scaling skill for axes too, but I cannot recall the name, and not sure if other weapons have one like this.
If, however, this game truly copied SaGa Frontier, unarmed is the strongest fighting type - that game had a skill called DSC, which was a hidden fist skill which only appeared when you learned 4 other techs and had them all equipped. It was the craziest shit in game - the Triple Thrust spear tech, one of the strongest, would deal around 6k damage. DSC went well over 20k. So Probbaly there is some unarmed skill here very powerful too.
