Elma likes melee attacks, both in her Arts arsenal and Soul Voice prompts. Considering how many enemies have abilities specifically aimed at punishing anyone sticking too close to their person, that does not seem like a wise approach to me.
While there are some Skills, Arts and Soul Voice prompts that boost her dodge chance, Elma is not a character you'd want to take enemies' aggro for prolonged periods of time. Keep the baddies off her and try to not let her kill herself by sticking to enemies she should not be sticking to.
There is also quite a bit of stuff in her abilities lineup that doesn't seem like something I'd be comfortable leaving in the hands of the AI. And lots of Arts that require TP to function. Granted, there are also some ways to gain TP faster than normal built in there, but...
Regardless of you choosing to control Elma to get the maximum perfomance out of her or leaving her in the hands of the AI, you'll be seeing her a lot, so you might as well get comfortable.
(Elma also happens to be the only character who refuses to show up on her spawn point if you don't have a free slot in your party, unless I'm severely missing something. Sneaky Elma.)
Lin has a gatling and a shield and a couple of unique abilities to go with them. Tatsu and his quest goal seeking ball comes with her as well.
One thing to note: there are two ranged Arts in the gatling branch that are cone AOE damage, which may either help you in a scrap or pull enemies you are not ready to handle, resulting in a surpirse TPK. Equip those at your discretion.
Shield Wall buff is recommended to be equipped at all times, IMHO, for everyone that can use it.
I don't think that leaving all tanking in the hands of the AI is something I am comfortable with, given how it doesn't use Taunt type Arts to get enemy aggro off party members that are in trouble reliably enough. Otherwise, a more or less universally useful character with some ways to Topple opponents and make use of their toppled state with thermal damage included.
Gwin comes across to me as a stopper in every hole kind of character.
He comes with the assault rifle + sword setup that has a bit of everything but does not excel in anything. There is a single-target Taunt and a melee attack Art that makes use of it, there are a couple of generally useful auras to choose from, some ranged attack Arts, a physical resistance debuff, an evenly divided set of Soul Voice prompts, etc.
To put it shortly, if you already have everything you want covered but still have a free spot, stick Gwin in it, he'll do fine.
Irina wields an assault rifle + knife combo, although the actual abilities available to her are quite different from what that combination would suggest.
Her arsenal could be roughly divided into four parts, out of which you'll need to choose two to fill up the Arts panel with. Healing, removing debuffs, casting Barrier and refilling other characters' TP gauge out of her own in one, general assault rifle stuff (ranged damage and a Topple ability) in another, Sleep effect and Brainjack under the latter two.
While Irina's skill selection suggest dipping heavily in one of the latter two, I have used her with the first two and was satisfied with her perfomance. That may just be because of me being hesitant to rely on mechanics that promise to be quite gimmicky, but I think she can work as your 'field medic' type of character without much trouble; the only issue is the Topple ability coming rather late in rank progression.
Doug loves himself some beam weaponry. He has a Topple ability and some finer things available late in his Art list, but, for the most part, Doug is here to shoot things with giant screw-off beam cannons and chop things into smoking mess with beam sabers.
About the only trouble with him I've had so far is that some of his ranged attack Arts can hit (and thus accidentaly aggro) quite far away. And, since Doug can't change classes, the only damage type increase skill he'll get is for the Beam damage type, so all those Thermal cannons and whatnot will probably less effective for him than for the player character.
- H.B. was the one I unlocked next.
He comes in a quest that's avialable after finishing chapter 3, if I'm not mistaken, and is equipped with an assault rifle + shield.
I have not found him to be particularly tricky to use, but I also didn't really notice anything special about him besides his shield attack that deals more damage to limbs/subsystems than normal.
He left me with a solid impression overall, my complaints about AI tanking notwithstanding.
- Yelv arrives in another quest that is unlocked at about the same time as H.B.'s
He is a second Doug. Well, I may be mistaken, but I didn't notice much difference to him. The Art, Skill and Soul Voice lineups differ, but I have yet to discern if I should use these two in different ways or not.
So I guess my recommendation would be to set him loose when you feel you don't have enough beams in your party.
Lao is a sniper rifle + spear kind of guy that gets unlocked during the chapter 4 story quest.
Some electric damage abilities, an all-party ranged accuracy buff and quite a bit of damage, more happy to call for ranged attacks than most. Perhaps the one party member I've had to pay the least attention to so far.
L is added to the party roster after you finish chapter 4. Comes equipped with psy launchers and a shield, with some good aura effects in both his Arts and Soul Voice.
L's first ranged attack is an AOE with Beam resistance reduction quality on it, so I guess L and Doug plus Yelv might like teaming up with each other (although I have yet to try leaning on a single damage type so much).
Becomes available in a quest after chapter 4 is complete. Plays like a slightly less suicidally inclined Elma (as in, more ranged, less melee), which is already a huge plus in my book.
Still has some trouble with having self-damaging Arts while already being on the fragile side and whatnot though.
Available in a quest after chapter 4 is complete. Comes with assault rifle + spear and unique Arts and Skills aimed at Skell piloting.
Has showed kind of lacklustre perfomance on foot so far, not sure what to say here.