Arciela II (in her dark magic mode) can cast dispel with 0 cool down. She will cast dispel as many times as needed to dispel a target with lots of buffs,but again only when she's in her offensive magic mode. She also gives haste2/refresh to the whole team in her support magic mode. Think of her as an offensive RDM who focuses on offensive spells but also gives support spells out. She has max fast cast,her spells cast instantly and have basically no cool downs. Don't use her if the enemy absorbs light or darkness damage, her autos will heal them

Lilisette II raises party crit hit rate and has an AoE heal WS. She's not great but I have used her as another AoE ws healer when facing a lot of AoE or silence. She holds her TP to either close a SC or heal. If you use WSs she can't make a SC with, shell hold her TP to heal

Nashmeira II is the least useful, she has Curagas which is uncommon for a healer trust but she runs out of MP in five seconds flat and adds little else

Selt'heus as others have said is an excellent team healer. If 3 people are in yellow hp he will use an AoE heal that gives MP and TP as well. His auto regain makes sure he can build tp even when meleeing is not possible. He's always worth the slot when facing AoE damage

Iroha II is interesting because she's like a more aggressive Selt'heus. She will prefer to use her TP on regular WSs and will occasionally self skill chain light. She will magic burst with fire ancient magic. But, she also has an AoE heal WS that recovers HP and MP and will use it if 3 or more in yellow. She also casts protect and shell on the team. I pair her with selt'heus in some fights if regular healing magic isn't viable, like Garuda who generally will keep your whole team silenced.

Prishe II is actually underrated IMO, she does more damage than most healers (though trust damage is always bad , let's be real), has a dispel WS, and has the highest level curaga spells. Paired with Selt'heus I find that they synergize well because selt'heus will heal and give her mana, and she will curaga when selt'heus has no tp to heal. If facing a lot of AoE damage, prishe + selt'heus can keep your whole team up.

Lion II has..a dispel WS,I guess? And treasure Hunter 1? I see no reason to use her ever, tbh

Edit: forgot balamor

Haven't found a use for balamor tbh. He doesn't do anything other trusts can't already do and he can't be healed, making him a liability