They are demonic shapeshifters who drive their victims to suicide, which they do through a mental illness, Metaphysical Awareness Disorder, caused by exposure to "Verbal information that is not desired to be known". While their motivations for driving humanity insane, or even if they have one, are unknown, it is blatant that they are malevolent in nature.
Shapeshifting: All types of Alternates seem to have some degree of shapeshifting, it is implied that this is a process that takes an unspecified amount of time and interrupting it would result in the Alternate becoming a Type 3 Alternate. the Type 5 Alternates have even found a way to shapeshift into objects and the type 4 are implied to be able to shapeshift into animals, as the Intruder has imitated a cat. Before shapeshifting, the Alternates are pitch black upon first entry to Earth. Psychological Warfare: All of them have the ability to inflict Metaphysical Awareness Disorder to their victims, a mental illness characterized by paranoid behavior, sleepless nights, panic attacks, and a suicide rate of 97% as of 1992. Intelligence: Alternates are shown to be quite intelligent they were shown planning before they attack, ambushing their victims, and with Type 3 Alternates using their deformities as advantages as shown at the end of "The THINK Principle".
Doppelgangers: They resemble their human counterparts and use that to psychologically assault their victims, drive them to suicide and then replace them. At most, they will have very minor, barely detectable differences in height or facial features compared to their imitated victim. Unspeakable: Previously Classified as "Detectables". The name that shall not be named. Flawed Impersonators: Doppelgangers that have not completed the process or had their process interrupted. As a result, they will be easily recognizable as Alternates, as they'll very often present features that are anatomically impossible for a human to possess, such as featureless faces or grossly disproportionate body parts. Tulpas: Unknown, presumed to be The Intruder. Mimics: They are presumed to be creatures that take the form of physical objects, they can be identified by their audible breathing.