I used several guides and tips from multiple sources during my playthrough of RoDD, and it became apparent that a large portion of them are straight up misleading. Here is what I had to find out on my own. Sorry if it's old news.
Characters only reveal parts of their background after taking part in a certain amount of battles. Character just has to be present at the end of each battle they can be used in, they don't have to do anything, but they must stay alive too. The commonly cited rules about them having to be healed after taking damage, about having to level them up and about them loosing trust when you don't talk to them and get them killed are a myth. It may seem that they stop talking to Max if you don't take them into battle, because they sometimes need two or more battles to continue talking.
Here is a spreadsheet of trust progression for every character in the game, under condition that each of them takes part in every battle: https://docs.google.com/spreadsheets/d/1kWF8SJP2ayBZHuXkddWkMCFmxUtytNlteARGidJt7cs/edit?usp=sharing
Please note that I am not certain about battles 5, 10 and 14 because they give no easy way to talk to the character before next battle starts.
In the final cutscene, whoever gives the speech and presence of characters is determined in the following way. The speech is given by whoever fully revealed their background and died the least. If nobody did, Nova will speak. If there are several characters who revealed their background and died equal amount of times in your party, the one with the highest level will speak. The Force members standing in the back will be in the following order: second-highest level in the middle, third-highest at the bottom and fourth-highest at the top.
Characters recieve over 10 exp only for healing those who are higher level that them. Exp seems to scale the same way it does when doing damage to higher level enemies.
I never found a full list of monsters affected by special weapons, so I made one.
Dwarfbuster - goblins, dwarves, lizardmen
Jagged Flash - giant bats, sea bats, gargoyles, belials, chimaeras
Zombie Charm - zombies, skeletons, ghoul, dullahans, skullforce
Doll Hater - mannequins, evil puppets, dire clowns, marionette
Elf Slayer - dark elfs
Duellist - rune knights, pegasus knights, silver knights, bowriders, horsemen
Mermanbuster - conches, shellfish, worms
Landslide - hell hounds, ice worms, cerberi
Raging Drum - artillery
Soulbuster - dark priests, master mages, Balbazak, high priests, Michaela, demon masters, Ramladu
Ogre Slayer - golems, minotaurs
Destroyer - jets, torch eyes, steel claws, Chaos, Colossus
Dragonslayer - blue dragons
So, every enemy in the game is weak to some weapon. I'm assuming wyverns have to be weak to Dragonslayer, but I don't know if it is even possible to get this weapon this early in the game. Elliot, Kane, Darksol and Dark Dragon have no weaknesses. It's strange that Soulbuster doesn't work on human Kane, but works on dark elf Michaela, while Elf Slayer doesn't work on Michaela.
Also, I want to point out that these weapons give double damage, not attack. So, if your attack is not enough to overcome enemy defense, you will do 1 damage with a special weapon, not 30 or whatever.