Slenderman, also known as Slender Man or Slendy, is a mythical being. He can be depicted in several different ways, but in the Runaway Kids universe, he is a tall, thin, white-skinned creature who wears a formal black suit. His face is flat and featureless. He has arms of an inhuman length that can stretch very far distances (Holly wrote that she "thought that they would go on forever"). Slenderman encounters cause paranoia and may cause insomnia, but ultimately afflict Slendersickness (blood spitting). Savannah and Bush Dwellers can create false Slendermen, which seem real, but eventually disappear. People who do not know about Slenderman before a false encounter can only be affected by Savannah Dwellers, as Bush Dwellers can only create beings from a person's fears. After a false encounter, a person will show the same signs as with a true encounter, but not as intensely. The main way to tell a difference between a true or false encounter is the Slendersickness. A person who experiences a true encounter will go through fits of coughing that causes them to spit blood, while a person who experiences a false encounter will not spit blood. True encounters are generally more frightening than false ones, and cause more serious injuries.
Water Dwellers experience regular Slendersickness, but during a false coughing fit, they spit sleet. Holly Axolotl experienced a false encounter with Slenderman on the 2nd of October, 2015, before she had any knowledge of the being. The encounter was created by Raponsal Lion-Cub, a Savannah Dweller. Holly, a Water Dweller, showed the signs of Slendersickness but did not spit blood. Instead, she spat sleet, which was liquid when she coughed it up, but became a tear-shaped ice cube as it hit the wooden floor.
True Slendersickness causes an Ice Dweller to spit very hot blood that can melt any soft substances that it lands on. It is advised that any Ice Dweller with true Slendersickness is kept away from valuable items and is given ice water to drink.
The coughing fits, paranoia, and possible insomnia generally subside after three to seven days. The only way to end the symptoms is for the affected to sleep through an entire night without a nightmare. This time varies, depending on how old and emotionally "strong" the person is. Even when the symptoms are gone, they may still experience frequent nightmares and remember the incident clearly.