Imariel is the fallen angel hosted by Polonius Lartessa. Imariel and Tessa are the second-oldest members of the Order; only Anduriel and Nicodemus Archleone are older. Archleone and Lartessa rarely work together, but when they do, it's never good. According to the Church's records, they did so just before the Black Death came to Europe.
The Fallen who follow Imariel go through bearers quickly. According to Michael Carpenter, they aren't kind to those they bond with. They are the monsters among monsters. Tessa chooses their hosts from among the downtrodden and desperate and will fall to temptation more rapidly.