Rhiannon

Short answer: Rhiannon is a 6-Star Beast Arcanist dealing Mental damage, and her own Insight text calls her an All-Rounder. She holds the squad's damage buff and the enemy's crit-defense shred at the same time. She is not a Reality carry, and building her as one is the most common mistake made with her.

Being Beast, she loses to Mineral on the Afflatus wheel, not to Star.

Attunement is the engine

Every card Rhiannon plays is paid for twice: the card does its printed job, and before it resolves it spends a stack of Attunement for something extra. She opens each round with 2 Attunement, so two cards a round are paid and the third is plain. A merged card still costs one Attunement, which is why merging matters more for her than for most supports.

At Insight 3 the leftover stops being wasted: a spare stack converts to Moxie at end of round, and she starts battle with Moxie +3 and 70% of her Raptor Kinship gauge charged. That is the strongest argument for pushing her to I3 rather than parking her at I2.

Bendith

Her Ultimate, Chirping Chorus, is a state change rather than a damage button. It grants Adar Llwch Gwin to the other allies and puts Rhiannon into Bendith for four rounds, which flips her two ordinary cards into reinforced versions. That is where her value lives.

The full build, including the Psychube case for The Soft Rain of Yesterday and why ATK beats Crit on her, is in the Rhiannon build guide.