Who is Door Portico from Neverwhere and what is their importance?

Door, as her name might suggest, is an "opener." She can open anything that might conceivably be opened, even people, creating passageways and throughways. Door can also open portals that cross a vast distance, allowing for a kind of teleportation. Using her opening talent, however, drains her bodily reserves, necessitating rest and sustenance. Door's capability is also metaphoric. She makes journeys possible by creating openings to new places and things. For Richard, Door acts as his doorway, not only into London Below but also into a journey of self-discovery.