Insightful Aid + Aid Lilya (R1999 Roguelike, Difficulty 15)
I'm in the middle of testing out some higher rarity characters as Aid. In general, it's a lot easier to run low rarity characters as Aid vs high rarities, mostly because high rarities tend to have complicated kits split between Incantation and Insight effects (and Aid characters can't activate Insight effects, causing some characters to not work as intended).
An easy example of this is Marcus. If you set her as Aid you don't inflict Annotate on enemies, and without Annotate she experiences a big loss in DPS. Some characters have it much worse (e.g. Rabies cannot inflict any Poison at all if he's set as an Aid character) though for a lot of characters, you "only" get the DPS loss. I'm trying to figure out which high rarities are least affected by this.
Lilya is surprisingly okay as an Aid character? Her Insights only affect her Crit rate, so her kit is mostly fine. Her Ult has a surprisingly good formula even at P0. With certain Catalysts (mostly Insightful Aid) you can easily cheat out Lilya's Rank 3 Incantations for very, very easy build up Thirty Seven's Follow Up Attack.
Unfortunately it needs to be another character running Aid Lilya, you cannot do Thirty Seven + Aid Lilya directly otherwise Thirty Seven doesn't get any of the Eureka from Lilya's extra attacsk.
Still Aid Lilya is pretty strong, especially with artefacts that activate off number of actions (like the stick that grants Ceremonious whenever an ally acts, or the Ring that gives stacks of Unsparing).