Berwyn is bad. Damage capabilities are bad, charm spell is barely better than a normal tease, Group heal is not good enough to make him the only healer, but the stone elemental summon is okay.
Quintillius is okay for tease builds. Both Bimbo Azzy and Dancer Cait are better for tease builds, but Quintillius encounter power helps nuking one strong opponent. For non-tease builds he's not good.
Atugia is extremly tanky, but she struggles at keeping her threat high. In my experience, she gets mostly ignored by the enemie and doesn't deal much damage.
For me, these 3 are the worst companions at the moment.
Arona can be strong, but is RNG heavy. Her accuracy is atrociously low in the Warhides/Chieftain/Beserker set. Out of these 3 sets I recommend Chieftain solely for "inspire wrath" which is a renamed blessing. Damage wise shes on par with standard Brint/Brienne. Where she really shines is her spirit totem set. Now she can actually hit stuff and can deal massive amounts of holy dmg. The holy shock power is a ranged attack which helps immensly against flying opponents. Basically no enemie resists holy dmg and demons have a 100% weakness against it. Arona can reliably one-shot imps and other demons.
Kyoko is a glass cannon. Very strong and very squishy in all her sets. Her weapon is a hidden gem as well. Not only does it deal pure holy damage, but scales with spellpower instead of attack power. This is especially useful in her Onmyodo set. In this set she can only deal fire or blight dmg, which demons are resistant to. 33% resist vs. fire and 75% resist vs. blight. Kyoko can deal between 50-60 damage to demons with a regular attack. This is about as much Brint/Brienne and Arona deal.
Azzyran is only mediocre in her standard set. She is more of a tank then a damage dealer. In my experience Brint/Brienne and Arona outperform Azzyran. Bimbo Azzyran is an ungodly terror who can win fights on her own. Not only has she two AoE teases, but she crits more often than not. Sadly she can't replace a healer, since her heal isn't strong enough and she doesn't have healers hand.
Brint/Brienne are the same in their standard set. What seperates them is that the pyromancer and winter knights set are mutually exclusive.
Pyro Brienne can only deal fire damage and is very good at it, but she has trouble fighting against imps and demons, because of the fire resistances they have. I suggest having the grease spell on your champ. The summon is strong, pretty tanky and usually generates alot of threat.
Winter Knight Brint is very tanky and has a nice synergy with his powers. Wintery bellow deals double damage against frigid enemies and Brint can set that up with entrhopic winds. Freeze solid is a stun and thats always good to have. However his normal attack sucks. He has terrible accuracy and his spellpower is far higher than his attackpower. So he relies on his powers for dmg dealing.
Cait is strong in all of her sets, but I would say that the witches corset is the weakest one. The only real advantage it has over the standard set, is that it has a resolve heal with eternal light. Problem is that most enemies don't deal meaningful amounts of resolve damage. This is just my opinion, but the buffs of the witch set don't justify the lack of offense.
I barely used Agni, so I can't say much about her.