First party ideas with Kevin(main)

[Rauber-sama]

So I'm about to start this game later today and I want to know which party would be better for me using Kevin as a main.
I plan to use Duran as a Tank/healer, but in the 3rd character I don't know who to choose.
I'm thinking maybe Riesz or Hawk, but what you guys think?
Also which classes would be recommended for a composition like this?

[Drake]

Depends on the difficulty. On normal and hard, it doesn't really matter. Item will cover whatever you're lacking. You don't really need a heal and a having tank not really a good choice because aggroing is meaningless, a buffer will be better. Usually you want to cover synergies between characters, not fill singular roles like tank/heal/dps.
This is an hack&slash, dps matters, having 3 heavy hitters that have some support spells is just better than having a full support heal/tank, a dps and a buffer. This is why some of the best parties are just angela/kevin/riesz or kevin/hawk/riesz going full damage with some buffs and debuffs to maximize dps.
Party composition is really a thing to consider when playing on extreme and no future (where you're getting oneshot if you're not careful).

About your party, Riesz is usually a good character to have, she does good damage and whether you go buff or debuff route, she's always good for a party. Since you don't plan on having duran go the elemental saber route, hawkeye might be a good option too, since his ninja route has elemental spells that also debuff. So you can cover both with one char.

[EbonySeraphim]

It depends. I think one of the most understated aspects of forming a party is understanding who you, the player, will control versus the AI. And the AI sucks for everything in this game except casting spells under given conditions. Neither Angela or Charlotte AI are entirely MP efficient so early game, if you're not on NG+ with a ton of money, beware on the magic walnut cost. That being said, burning magic walnuts and casting the healing spells is more efficient as soon as you hit your first class change and by second class change you should be pretty easily managed. If you're dead set on Duran being a healer/tank AND you don't plan on controlling your non-main fighter at all, Angela AI control will put in the most damage in. More than even the stronger damage dealing fighting set to the most aggressive settings.

A note about Angela AI: you need her to cast on the most aggresssive settings. If you have her set to any sort of hedging about when to cast based on mana or who needs help, this is where the AI seems to do dumb stuff and just sit there inexplicably. She will drain her mana, and sometimes cast expensive skills at the end of battles but if you don't want choppy play micromanaging every cast for her, it's a sacrifice you have to make. If you want magic walnut usage to go away, Grand Divina class for her 3rd job is pretty darn sick. It has a locked ability called "Refresh" which gives you back 15% mana at the end of a battle. Any job either than Rune Maiden (unless you know when to use Rune Maiden) will be very effective for Angela, even when played by the AI. Even Rune Maiden is strong, but by late game, there are a fair set of enemies that are immune to all of the basic 4 elements, some immune to just about any element, and Rune Maiden will literally have no spell to harm it.

[darth.crevette]

My Kevin playthrough was him in a dark class (full unga-bunga mode, can't remember which of the two classes it was though, or if one is considered better than the other), Carlie - mostly for story purposes - as High Cleric (lots of undead in their story path, and physical buffs on Dark Kevin is great), and Dark Riesz, probably Fenrir Knight? for debuffs.

Fully buffed with Strength Up and elemental saber Kevin, against enemies with Def Down? Yeah, works great. Not sure about the higher difficulties though.