As Devils are shape-shifting, they can decide to look like anything they choose. However, they cannot use the magical abilities of the creature they transform into. An example is a Devil cannot change into a specific Wizard and have that Wizard's specific magical abilities. It is unknown how they choose what thing they transform into.
Devils do not reproduce, instead, they copy themselves.
Devils are cruel and cold hearted. They allegedly act as assassins to Mortis, travelling between the two realms, the Abyss and Mythos, eliminating potential threats. However Devils are not nearly powerful enough to complete more important tasks like breaking the barrier that prevents Gods from passing through the two realms.
Devils are very intelligent. They use complex battle strategies to eliminate targets.