Master Deimos is the apparent head and public face for the Hooded Wings guild, though he seems to leaves daily assignments to Grimm. He can be found on the balcony of their guildhall in the Outer City. Scarlett should see Master Deimos about joining the Wings.
Master Deimos sells potions as well: Health Potion, Mental Energy Potion, and especially the Storm Potion, which temporarily increases running speed, enabling faster deliveries.
Master Deimos also converts herbs into potions: Dusk Whin into Health Potions and White Ivy into Mental Health Potions (Note that Scarlett can make these potions, too, provided she has the Alchemy skill and the necessary Alchemy Phial)