Alright! Even though the game's been out a while, I thought I'd give a little bit of a primer on roster setups. There are roughly 7 billion cards to unlock in Rance X (1 for every human being on Earth) but making an active roster of 7 can sometimes be difficult. So I thought I'd give a little advice on what I look for when creating a team. You may want to use the Alicesoft wiki for reference if you have no idea who these people are.
This is mostly for bosses/difficult fights, as you'll probably be focused on leveling up cards for easier fights. Also, this is mostly for when you've unlocked 7 spots - the principles of team composition still apply, but you're much more limited in what kinds of contributions you can prioritize (you may only have 4 slots!). Anyway!:
2-3 damage dealers. This is all about maximizing AP usage. Having a team of all damage dealers makes no sense: if Character A hits for 20k damage for 2AP and Character B hits for 10k damage for 2AP, unless they've got some passive abilities, using AP on Character B's damage is a waste. This is going to vary from mission to mission, since element bonuses and faction bonuses can give you anywhere from 50% to 150% damage boosts and should absolutely be taken into account. Other than Rance, this means there really isn't one or two top damage dealers. Still, you'll see some familiar faces in this role: Rance, Rolex, Rick, Full Kalar, Kenshin, anyone from the Demon Faction once they've powered up. This also largely depends on your most powerful factions: if you don't have many cards from Zeth, they won't be hitting very hard. Also, it depends on who they're competing against from the same faction: Kenshin is a pretty good hitter, but unfortunately she's competing with THE KILLER DANGO.
% based attacks. For early in a fight, when enemy hp can be extremely high. Basically, poison and curse. Poison hits for 5% of an enemy's current hp every turn and curse hits for 20% of an enemy's current hp after 3 turns (+1 turn for every time you've used it). There are a few options on who to use here, but usually your best options will be Kou for poison and Pastel or Vivid Kalar for curse. A special mention goes to Papaya Server, who can double poison's effectiveness to 10% per turn. The usefulness of these abilities wanes as the fight goes on, so it's probably worth switching these characters out later in the fight.
Enemy buff removal. Some fights can revolve around enemy buffs (flight, Babolot's eating habits) or just make things much more difficult (+100% damage, +100% defense, etc.). You've got plenty of options, some of whom remove buffs as a skill or as part of their attack: Miracle, Magic, Arlcoate, Sachiko, Urza, etc. Oruoure and Crane will remove up to 4 buffs with their skills. Who you choose among those will depend on other factors but it's usually worth it to have someone on hand who can remove these at a moment's notice.
Team buffing. Lots of options here as there's lots of different buffs. Tactician buffs raise damage by 10% per stack (like from Kou or Arlcoate), defense buffs raise damage reduction (like from Caesar), attack skills raise attack damage (like from Kenshin), healing skills can raise the auto-healing rate (like Horus Queen), etc., etc., etc. There's too many to name and what you choose to have on hand will depend on what challenges you face in the fight, but it's almost always a worthwhile use of AP.
Passives. This gets its own section. You'll only have so much AP to spend, so having someone who can contribute by doing nothing but sitting there is a huge boost. Here's some examples but again, there's too many to name them all: Sachiko gives you a +20% defense boost, Cafe gives you a +20% attack boost, Papaya boosts the rate of auto-healing, Arlcoate give you tactician bonuses, Nunuhara Cabbage makes element bonuses happen every turn, etc., etc., etc. Take note of characters that can contribute even when you ignore them.
Healers/debuff removers. Not an immediate priority in most fights (and if you're kicking ass you won't need them at all), but as the fight drags on you'll need to know who you can switch in if low hp becomes an issue. These same characters can often also help you with a second issue, enemy debuffs. Most fights won't feature these but some can revolve around them: Medusa can permanently turn characters to stone, Hawzel can hit you with a debuff that makes fire damage do extra, etc. You've got a good amount of options: Sill, Japan's Mikos, Agireda, Copandon Dott, Mi Lordring, even Rance's maids if you get desperate. Ruberan Tser can remove debuffs at no AP cost (and she's not the only one). You won't always feature these characters but you'd better know who they are when things go south.
Wild cards. Again: 7 billion cards to choose from. Plenty of them have fairly unique skills that may be helpful depending on the enemy. Eleanor can put enemies to sleep (even bosses). Ninjas can prevent enemy actions. Crook can put up a damage shield against 1 enemy attack. Atago McCart can give you extra AP. 7 billion options. Go nuts here.
Most Important: PEOPLE WHO CAN DO MULTIPLE THINGS OUTLINED ABOVE. This is what is going to separate your top characters from your average/mediocre characters. For example: Cream has a 20% chance to passively give you a tactician bonus. She can also actively give you one at the cost of 1AP. Compare this to Arlcoate: she has a 40% chance to to passively give you a tactician bonus (60% in one of her alt cards). She also has an attack that removes enemy buffs (it's even ranged so it'll hit flyers). Arlcoate checks three boxes well (passives, buffs, remove enemy buffs). Cream checks one/two (kinda) poorly (buffs). This is the most important part of team composition - who is helping you in multiple ways.
Extra Bonus: Naked cards. Because why not?
Here's an example of a generic full team that checks many of these boxes:
Arlcoate (passives, buffs, remove enemy buffs)
Atago McCart (Wild Card extra AP, has a useful naked card too!)
Dark Heron (Wild Card enemy attack prevention, damage shield)
Kou (passives, buffs, % based damage)
Pastel (Damage on White Element, % based damage)
Caesar (Damage on Red Element, Passives, Buffs)
Rance (GAHAHAHAHA)
On hand for healing: switch out Kou for Mikos/Agireda once enemy hp so low that poison no longer worthwhile.
Honestly, there's a million ways to put your team together. I wrote this up because there are certain things you'll want to look for (and if you don't speak Japanese, this becomes harder), but tinkering with putting your team together is half of the enjoyment of this game. Just remember to play around and have fun.