hikikomori
Japanese word for describing socially withdrawn people (usually Japanese), who rarely leave their house or even bedroom. The official definition is someone who engages in this activity for at least six months.
Anime characters that are examples include Satou Tatsuhiro from NHK ni Youkoso! and Sakurada Jun from Rozen Maiden.
A related problem is that of NEETs, who often become hikikomori as well. However, it is important when tagging to note that NEET and hikikomori are distinct, if related concepts.
For the Youkai Watch character, see Hikikoumori.