Man, trying to remember everyone from 'Arsenic Blue' is a real test of memory. The core is definitely Gwenifer, the protagonist whose discovery of the old arsenic-based dye formula kicks everything off. Her brother Leo is the skeptic turned believer, and their dynamic drives a lot of the emotional weight.
Then you've got Professor Alistair Finch, the academic with shady connections to the historical dye guild, who serves as both mentor and antagonist. The real scene-stealer for me was always Elara Vance, the enigmatic chemist from the rival family. Her motivations are so ambiguous right up to the last act. Oh, and you can't forget Inspector Hayes, the dogged police officer who thinks the whole thing is a modern-day murder case, not a historical mystery.