Do you want pure minmax or just fun builds that are viable on core/hard?
With Aru you can't really do anything other than ranged dps with a longbow since she already has bow feats stacked up. Personally I just like 20 ranger since you get lots of ab, but it doesn't matter much if you have a bard and community domain. If you want sneak attack on her I'd recommend going into slayer rather than rogue for the full bab progression.
I usually turn Daeran into a fire blaster with the ring you get from the healers tent vendor in act 2 that gives fire spells. Grab that and 1 level of crossblooded sorc with two fire bloodlines and he'll be good at blasting in between his heals. If you go ember you do the same thing with her instead so she can blast between hexes. I've also seen second mystery bones to make Daeran double as a necromancer nuker which is nice but I think the fire focus is better.
Paladin/bard is probably the best Seelah build for the frontliner but I find it pretty boring and annoying, since you need her to strip every time she casts, Paladin/Skald is also an option but idk shit about Skald. You're much better going pure paladin over paladin/fighter since Seelah will never be a powerhouse on her own, she'll always be a support character. Pure paladin is good but she'll be super squishy so you either need to get her on a horse or give her a reach weapon, I prefer a reach weapon since mounted combat sucks. Then just stack cha so she can share her ab with the rest of the team. Mark of justice is the best ability in the game.
Woljif is the best of the three dual wielding companions since he can actually get decent ac, it's a shame he starts at level 3 so you can't make him an arcane trickster. 4 rogue/16 vivisectionist is very good, gets full sneak attack and excellent ac and damage, and he can get infusion so he can give personal buffs to teammates. Full rogue works the same but is strictly worse imo since he'll have a lot less dex and ac. I've also seen sword saint/duelist Woljif builds floating around to stack int to ac but again they're strictly worse than just going vivisectionist since they sacrifice a ton of damage and spellcasting progression. There's also the classic 10 eldritch knight/2 eldritch archer/8 eldritch scoundrel ranged build, this has a few wasted feats and is imo pretty weak next to his vivi build, but it works if you want a ranged Woljif.
You can keep Camellia the entire game, just do her quests. The only reason you can't keep her is if you kill her. Personally Camellia is my favourite built companion but she also has the least variety, she has 1 pretty clear build with no variation. 16 shaman, 2 instinctual warrior barbarian, 2 fighter. This gets you her casting stat (wis) to ac, extra feats, and level 8 spells. Her shaman archetype allows her to enchant her weapon with 3 elements, combine with elemental barrage mythic feat and you can do 15d6 divine per attack at mythic rank 10. Or skip enchanting her weapon and just cast geniekind four times for 20d6 damage and keep the weapon enhancement bonus from the enchant ability, both work. If you're willing to buy and craft mage armour potions you can take second spirit waves and archmage armor for seamantle and 14 ac mage armor potions, this is the way to get her the most ac. If you don't want to deal with potions you can just go second spirit wind and use air barrier to get a similar effect to archmage armor, and grab waves as a third spirit for seamantle. Since she can have frightful aspect she can activate shatter defenses for her whole squad, deal a ton of divine damage on every attack, and have ac in the 80s. She's by far the best martial companion in the game and the companion that can get the highest ac, she still can't tank unfair but on core she'll be basically immortal. You'll want to drop her shield at some some point to use the crane line of feats, the two fighter levels are to get enough feats for all of the crane line. She's even better if you have an alchemist to infuse her with the shield spell. Make sure to take the iceplant hex and icy protector from the Defenders Heart vendor for an extra 4 ac.
And there's no need to worry about custom characters, even on unfair the companions work just fine as long as you have a good mc since the world revolves around you.
