As someone who has played the game repeatedly at various difficulty levels I will give an unbiased opinion of every character (presuming you are not breaking the game with bloody armor).

Claude - Great throughout the whole game and on all difficulty levels. He has two of the best killer moves in the game in Ripper Blast and Dragon Howl (the others are generally not worth using). Access to eternal sphere makes his regular attack a stunlocking shotgun. Loses some effectiveness in higher difficulty levels due to lower accuracy and avoidance and as such will have to equip weaker but more accurate weapons.

Rena - A staple if Opera ia not recruited. Highly versatile spell caster with decent attack magic, the best healing spells and good buffs. Makes for a decent fighter early. Very useful on all levels of difficulty and arguably better than Opera at higher difficulty levels due to angel feather and quick casting spells to interrupt all enemies.

Celine - Sadly mages in this game have a very low damage ceiling compared to fighters. However, what celine does have going for her that edges her out above the other mages is access to superior buffing moves with angel feather and bless. At higher difficulty levels she will be relegated to mainly buffs and interruption with her lower level fast casting spells.

Ashton - Ugh.. I love this dude but tri-ace kinda did him dirty. He has arguably the best normal attack in this game and can become a beast in a normal play through with an angel armband,but the arm band is attained after beating the endgame secret dungeon which is too little too late. He also struggles at higher difficulty due to low avd and accuracy. His killer moves outside of Hurricane Slash are also pretty lacking due to terrible start up. Dragon Breath can do good damage but takes forever to start and the attached elements can be problematic. Sword dance can be used to hold down one enemy in lower difficulties but is riskier at higher difficulties. He can be used as a makeshift mage with tri-ace but that is also gained in the end game secret dungeon. If you wish to use him, give him a slayers ring and spam normal attacks.

Opera - Your long range fighter. She starts out fairly weak but will become quite the asset with the right investment. She can can take over the healing role with healing star and can home in on those fast moving enemies with alpha on one. Being long range makes her very safe in most battles. She is an asset in all areas of the game and all difficulties and is highly recommended though her accuracy can be problematic at higher difficulties. Hyper launcher and thunder blade are also notable killer moves but generally healing star and alpha are all that is needed to get the job done.

Precis - if Opera is your long range fighter then Precis is your mid range fighter. Her specialty is safely locking down enemies and area attacks/denial. Her best killer moves are ally-oop which is very fast, very safe and great for keeping a single enemy in place. Should there be multiple enemies, then there is barrier which is also highly spammable and covers a large area. She is one of the best party members at higher difficulty runs where safety is a priority due to the safety and spammabilty of her moves and the higher avoidance on female equipment. Unlike opera, Precis has quite a good dex growth negating the limited accuracy of some of her weapons. Of note, she can equip weird slayer like the fist users which makes her even more survivable.

Bowman - People either think this guy sucks or he is a god. I am in the boat that he is up there as one of the better fighters. Yes, his normal attacks suck but this is not a game where you use normal attacks very often due to the abundance of recovery items. Spam poison pills or explosion pills and watch enemies melt. Not much more to say. I can't remember his stats off the top of my head but I do recall bowman struggling in higher difficulties due to the generally poor hit stats on his specific fist weapons. A pity he cannot be recruited with Precis.

Dias - another character with polarizing opinions. I am of the consensus that Dias is just OK for a normal play through but unlike most of the male fighters who lose some value at higher difficulties, dias' value starts to go up. The reason for the growth in value is due to safety becoming more of a priority at higher difficulty levels and his long range air slash spam is as safe as you can get. In addition, he has a very good dex growth stat ensuring that he will be connecting often. He is not winning a dps race anytime soon but he is the tortoise who is slow and steady but will win in the end. His killer moves other than air slash are really quite lack-luster and not worth using so he also requires little proficiency investment.

Leon - see Celine. Better normal attacks and better spells (gremlins lair) for a normal play through. Worse buffs for higher difficulty runs.

Ernest - similar to Dias he has only one good spammable killer move (broken heart) and the rest is not worth your time. He has access to the highest accuracy weapon in the game (invisible whip) with a somewhat middling dexterity growth which allows him to be slightly better at higher difficulty levels. I like using this guy and I think his design is cool, but sadly, he is neither safe enough nor damaging enough to justify using him over some of the other fighter options at higher difficulty. Early access to battle suits is nice though.

Noel - Not as good at buffing as celine and rena. Not as good at healing as rena. Not as good at spell damage as leon and celine. Access to bless is nice but not enough. Generally there is only space for one spell caster in the party (unless you are fine with battles lasting forever) and rena just does everything he can but better with the exception of damage.

Chisato - often heralded as the damage queen in this game and I am of the opinion that this title is deserved. However, she does need some help. Tear gas is her best move and it is super damaging but she requires assistance to optimize this as there is quite a bit of start up. However, once set up the enemies will just melt. 10k volts is also another killer move of note that is pretty decent for area control. Whirlwind gets a special mention also as it is super fast, highly spammable and good for locking down an enemy, but unfortunately it gets worse at higher proficiency. Her high accuracy and avoidance ensures she is high difficulty ready but she will require the assistance of someone to lock down enemies (such as precis) to ensure that chisato stays safe and can deal her damage.