Alan is a divorcee who had been fired from his job. Subsequently, due to having plenty of time on his hands as he put it, turned to masturbation, pornography, and one night stands. He had also been having dreams about Lusst'ghaa, which he had been seeing a therapist named Dr. Austerlitz about.
Lust from Beyond: Scarlet (Spoilers)
Alan is lured to the abandoned Eden Theatre by Rhea (who teases him with promises of sex). Captured by the Scarlet Lodge, he is selected as their Seeing One, and subjected to trials of being sent to Lusst'ghaa.
In the first trial, he travels to Lusst'ghaa and leaves a Trail in his wake. He is able to use this to return. During his subsequent visit to Lusst'ghaa, he discovers someone else's Trail, evidently that of another Seeing. He uses this to return to Earth and escape the Scarlet Lodge.
Once free, he relates his story to Dr. Austerlitz. Austerlitz is revealed as the head of the Scarlet Lodge, and thus he is swiftly recaptured. Alan is sent to Lusst'ghaa several more times, until his exhaustion leads a frustrated Austerlitz to send Tim the fat man to beat him. This seems to trigger his abilities once more and he appears to escape to Lusst'ghaa.
Lust from Beyond (Spoilers)
In Lust from Beyond, the player can find him enveloped in an Enthralled in the Chamber of Change. He is unmoving and it may be surmised that he experienced the heights of sexual ecstasy until he died.