Deaths are humanoid beings which are always male.
When a human is born, their Death is also created. The behaviour of the Death during life depends on the attitude of their human. In at least one world, the humans find the presence of their Death comforting and so it remains visible. In Lyra's world and Will's world, however, deaths are feared and so they hide from their humans until they die. They are quiet and are able to blend into the background with ease.