I'd say it largely depends on team composition, because the characters are not created equal.
Do also keep in mind that normal attacks are generally useless and items such as mp potions are extremely useful.
Princess Yustie/Justy is an amazing general support throughout the entire game. She can heal, provide a team wide one time status immunity and revive. She can also buff physical units. Her greatest strength however lies in her lvl 20 and 30 scepter skills, allowing her to cheat the teams mana cost and supply a strong offensive buff for the turn respectively.
Wein/Wayne may initially seem like a DPS, but ideally she should rarely serve that role outside of the early game, because while she may possess many a damaging skill, she gets her strongest skill at lvl 1 in the form of a party wide substitute. The substitute has boosted priority and a fairly low cost, so you can practically spam it every turn and free up the rest of the party to do their job. She's fantastic at all points in the game, provided she can't get taken out in one turn.
Iris is a generalist mage. She can either dish out some damage or do some healing. She can party heal and whereas Yustie can only prevent status, Iris can actually remove them. On the damage side, she can put in work in the early and mid-game and then gets outscaled by others in the late, due to her lvl 30 spell being a bit iffy to set up.
Noel is a great late game mage, but for a majority of the game she's basically dead weight. In early and mid game, she has atrocious utility and her spells don't deal enough damage to justify her cost. Late game she allows for consistent takedowns of most if not all bosses in 2-3 turns thanks to Ars Magna at lvl 50. Only real problem is getting her there.
Sanya is your drop hunter and a prime DPS for most of the game. Her drop rate passive helps with farming and at lvl 20 gets a damage skill that guarantees the drop if you kill with it. She gets an array of status and special bullets. Her offensive stance grants her sizeable buffs but leaves her defense paper thin, but this is easily circumvented with the help of Weins substitute. Once lvl 20 rolls around is when her damage truly takes off with her triple tap skill, allowing her to take care of lone targets with relative ease. You can bring her for any part of the game.
Lean is a counter tank. She tanks by taunting instead of substituting, which only means she has a higher chance of being targeted and as a result may leave one of your less durable party members open at unfortunate times. For this reason i didn't end up bothering with her too much.
And then finally we have Pinel. She's got some healing capabilities, but i personally find her less effective than both Yuestie and Iris in this aspect. Her offense is also sort of iffy... At least until lvl 40 where she gets a similar buff in offense power like Noel. Pinel after her lvl 40 skills is an absolute burst Demon. She gains a passive that doubles her ATK if shes at half or below and her level 25 skill conveniently makes her take half her current hp as damage, easily setting up the condition. Her other lvl 40 skill is a 5 hit skill with priority and a guarenteed stun, but does sport a 5 turn cooldown.
For most of my run i was running Yustie, Wein, Sanya and Iris and then eventually slotting in Pinel or Noel in for late.