It sounds like your game is pretty story-driven,with well-defined characters, instead of blank cardboard characters the player customizes his/herself.
It has been pointed out that you need to flesh out your story, and thereby your characters, and their class distinctions will follow.
But if you're having some problems working out what those differences are, here's some notes:
PHYSICAL CLASSES:
Berserker: these guys are pure attack. They are often of average speed at best, and have low defense. They usually can't equip heavy armor, and are often limited to more "crude" weapons like axes and hammers. Sometimes they have a default Berserk state which ups their attack power and prevents them taking orders, but you can make a skill which induces this state, instead.
MARTIAL ARTIST: much more finessed than a Berserker. High attack, high speed, low defense. Sometimes, because of their spiritual nature, they may have a fair magic defense. They're usually limited to cloth armor, and claws or gauntlets as weapons, sometimes no weapons at all. Without the modifiers equipment adds, they have to rely on their base stats. You could give them "chi" spells feuled by TP.
KNIGHT: more middle-range stats, but they get to equip more weapons and armor than anyone. Higher base defense plus equipability of heavy-duty armor and the natural story points of making knights valiant soldiers of honor, makes them natural tanks. Give them a "cover" or "protect" skill so they can take hits for their frailer allies.
THIEF: RPG maker has no "steal" skill, so I recommend using a script like Yanfly's so your thief can actually steal items and gold from enemies. Thieves are fast, with high evasion, and are adept at status moves. They can't hit hard, but their typical dual-wield weapon slots allow them to hit twice in one turn. You can also make thieves necessary to pass certain doors, requiring them to "pick locks". You can give them skills based on stats like agility and luck. They usually only wear lightweight armor, and small weapons like knives, claws and boomerangs.
SAMURAI: a sturdier version of the martial artist, samurai's can equip light armor, katanas and sometimes bows. A lot of their skills involve taking on multiple enemies and single-hit KO's. Pretty culture-specific, so make sure it fits your story.
MAGICAL CLASSES:
WIZARD: usually possessing the lowest defense and highest magical attack, wizards focus on attack spells. They call down rains of fire to blast their enemies, but are pretty vulnerable while casting their spells. They can usually only wear robes and cloth armor, and fight with staves or wands. Almost all their skills will focus on offensive spells, with maybe a few status moves thrown in.
PRIEST: the flipside of the wizard. still fairly frail, but their recovery and stat-buffing spells make up for that. Equip-wise, they're similar to the wizards, but sometimes can use hammers, too. In addition to their healing spells, they often have demon and undead-smiting powers. *note: what sort of religion does your priest follow? This can influence the elemental alignment of her spells.*
SAGE: sages are usually sort of nebulous, nonspecific spell-casters. They mix offensive and defensive spells with frail stats and limited equip options. Sometimes they can equip knives, if you make more witchy, but some games have them equip books as weapons, though these usually have little attack power. Some games give them a "sight" skill which allows you to check an enemies stats and weaknesses, but this requires a script.
DRUID: a much tougher version of the sage, druids can attack and heal, but are usually limited to "natural" spells. They can also use weapons with more efficacy, but lose some magical power in the trade.
HYBRID CLASSES:
PALADIN: holy warriors. They have the divine attack spells of a priest, but not the healing power. They can use heavy armor like a knight, but are usually more limited in weapon choice. They are also usually slower than knights, as well.
RANGER: specialists in long-range weapons like bows and boomerangs, they hide at the back, where their low defense won't get them in trouble. Because they're outdoorsy types, they often have a few nature spells under their belt, too. Lightweight armor is the best for trekking through the bush.
MEDIC: a knight with some healing skills, medics are limited to lighter armor and weapons, so they can tend to their injured allies, even though they lack the full powers of a priest.
SPELLBLADE: warriors who can enchant their weapons with elemental or magical properties, and sometimes have a couple of basic attack spells, too. They are usually limited to swords and light armor.
These are just some of the more basic types, and there are definitely several others with tons of variations.
Hope this helps!
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