Shellder are a pokemon which seldom move, carry their homes with them, and their round, black bodies are easily mistaken for a life lived in literal darkness. This has made them the perfect metaphor for the pokemon world's hikikomori community, and indeed among these shut-ins and the message boards they frequent, the name "human shellder" has become a common term of reference.

Furthermore, Shellder are a pokemon which require little care. Provided they have an ample supply of food and a water tank to remain in, they are easier to care for and longer-lived than most fish, and indeed than most grass pokemon as well. They are, however, extremely unresponsive to human contact, and paradoxically will only truly bond with the most asocial of trainers.

When a hikikomori (or indeed anyone who has truly bonded with their Shellder) dies in their home – of suicide or drug overdose or wasting away or even old age – the Shellder will die as well. Some say it is the Shellder's death which kills the trainer and not the other way around, although this is not commonly believed; it is difficult to determine, as hermits and Shellder have similar lifespans. When this happens, the Shellder's spirit will float out of its body and turn into a Gastly. Once transformed, it will protect the peace of its trainer's soul by scaring away all intruders until it is captured or the room exorcised.