The mimetidrene /mɪˈmedɪdriːn/ is a black, eye-covered stranger. Its skin has the appearance of being soft and smooth, and its eye markings glow in dim light. Though the mimetidrene appears superficially solid, any touch causes its body to break apart into a dark mist that feels vaporous, and leaves no residue behind. The stranger's form is filled with structures that are not unlike brains, and which are visible only when the body is sliced with a clean split.

The mimetidrene appears in any quiet location with long halls and dim lighting, such as closed-down schools, unused office buildings, and private-access underground roadways. It prefers nocturnal hours, and appears during the day only 18% of the time. When it first appears, it starts off as a loose, dark cloud, which coalesces over the next several hours. It never forms near another of its kind.

The mimetidrene's disposition is deliberate and routine. It appears unaffected by stimulus in its vicinity, and travels through its environment with careful steps.^㇎ It stops at hourly intervals, during which it remains stationary for either thirty, or sixty seconds. Upon pausing, its back legs continue to move forward for several steps, causing its elongated body to take on a temporary curl or wave.

When the mimetidrene reaches a fork in its path, it duplicates its body to split into as many separate individuals are necessary to travel down each path.^꒽ The mimetidrene continues to duplicate itself each time it encounters a junction, even if it has already duplicated itself at that junction in the past, and upon reaching a dead end, turns and doubles back on its route.

A mimetidrene can only travel for 1,987 feet before it slows down, stops moving, and fades away. This distance is reset, however, following each duplication, and thus, in an ideal environment, a single individual can persist for an indefinite period of time.

When a mimetidrene meets another of its strain, however, both individuals stop moving, and fade away.

Upon encounting a sensitive, the mimetidrene always moves past, around, or through them, disrupting its own body to do so. Though it briefly pauses, it does not otherwise react to any action on the sensitive's part, and can be destroyed with relative ease^Ꭾ and no deleterious side effects.

Its path can be halted, however, when a sensitive stands to face the front of the stranger, palms spread, and arms raised at a 25° angle. This specific action causes the mimetidrene to stop moving, and recite two numbers. It first states a number corresponds to the number of sensitives killed by strangers within a city since the beginning of the infestation in which the mimetidrene occurs, followed by an apparently random number between 1 and 999,999,999.

Although the mimetidrene is not physically harmful, 0.7% of sensitives experience persistent, intrusive thoughts of a specific number between 1-999,999,999 following an encounter. An affected individual finds themselves unable to resist the compulsion to count to this number, and does so over and over. Although this effect is temporarily disruptive in the case of large numbers, it persists no longer than four weeks after the encounter.

After a mimetidrene walks 1,987 feet, it stops moving, and repeats a number that corresponds with the number of times it has duplicated itself; an unduplicated mimetidrene states the number "zero" with fearful regret in its otherwise monotonous voice.

Upon stating this number, the mimetidrene dissipates into a black vapor. It leaves a stain on the walls around it that washes off with water and a washcloth.