Marissa Fittes

Marissa Fittes is the founder and head of the Fittes psychic detection agency in the Lockwood & Co. series of books.

The Problem
Due to Marissa's meddling with the "Other Side" helped with information by a ghost named Ezekiel she accidentally stirred up the spirits of the deceased and caused what is known in the books as "The Problem".

The Problem is, in the simplest terms, ghosts returning back to the living realm through weak points. They do this whenever a living person agitates them by entering their realm, which Marissa did.

Ghosts can return without being "Stirred up" however this is very rare.

Immortality
Using the knowledge she'd gained from the other side, as well as help from Ezekiel, Marissa managed to turn herself into an unprecedented type of spirit, still technically alive but without need for a body

After doing this, she faked her own death and waited many years while her daughter ran the Fittes agency.

Once her grandaughter, Penelope Fittes, was old enough to possess the company, Marissa possessed her and drove her out of her own body. She ruled many years under the guise of Penelope Fittes until the explosion at the Fittes agency headquarters caused her old body to be destroyed, severing her tie to life.