Alan's Predecessor is the nickname of an unknown individual, sent by the Scarlet Lodge to explore Lusst'ghaa. This individual was the Lodge's Seeing at some point before their capture of Alan.

Lust from Beyond: Scarlet (Spoilers)[]

On Alan's second trip to Lusst'ghaa, he is unable to get back to the Trail he created that would have led him back to Earth. Fortunately, he locates a Trail left by another Seeing, clearly one who came to Lusst'ghaa and (for whatever reason) never returned. It is unknown how many Seeing were used by the Scarlet Lodge before Alan, but the presence of that Trail confirms one at least.

Speculation (Spoilers)[]

A Seeing trapped in Lusst'ghaa, one of Alan's predecessors.

Since the Trail remained in Lusst'ghaa, then whoever made it during their arrival must still be in that realm (be they alive or dead). During his first trip, Alan does indeed spot another human, alive but trapped and being ravaged by tentacles. This woman may or may not be the one who left the Trail that ultimately leads to his escape, but she is certainly another Seeing.