Hikikomori is a Japanese term used to describe a social phenomenon of a young person withdrawing from society to live in isolation for a period of at least six months. Hikikomori are generally associated with either feelings of fear or apathy toward the outside world, which become gradually more excessive over time, and tend to have an affinity toward typical otaku culture such as manga and anime.
Within the Planner Scenario, the sheer amount of dedication it takes for a person to remove themselves from society has resulted in the qualities that make up being a hikikomori being considered a Skill Level. While the average person is capable of not leaving their home for extended periods of time with few social contacts, a person who possesses the Hikikomori skill is capable of removing themselves from society entirely and live in complete isolation in remote and often dangerous locations. In battle, a person with a Hikikomori skill level is capable of instantly withdrawing at random, making them highly unreliable allies. While having a Hikikomori skill level doesn't guaranteed that a person will become a hikikomori, they are at a greater risk of becoming one than most other people, and must try to avoid large amounts of stress and over-stimulation as a result.
The only person known to possess the Hikikomori skill is Aten Gnu, a promising mage who lost all of her motivation when she awakened to it. Before meeting Rance during the events of Rance Quest, Aten spent years of her life living in the deepest reaches of a monster-infested cave, with her existence only being known through rumors passed around by word of mouth. As Aten only possesses a Hikikomori skill level of 1, it is unknown what the capabilities of a person with a skill level of 2 or 3 are, a question that will go unanswered forever, as they have likely completely erased all traces of their presences from existence.