He first appeared in Log 12.
Several months prior to the beginning of the Dark Harvest series, Greg was followed by a figure wearing a white mask. This culminates in KindVonDerRitter breaking into his house while Chris and Alex visit him. Taking Chris and Alex's advice, he begins filming himself after the traumatic event. Soon after, he starts hearing strange chanting emanating from the building directly across the street from his home, but he is unable to decipher the source.
On March 15th, 2011 (interestingly enough, the Ides of March) Chris finds Greg dead in his house as the Slender Man watches nearby. Detective Amsel, a "police investigator" who receives Greg's corpse DOA, states that "the Tall Man" had killed Greg. However, due to Alex and Chris' skepticism of his authority, the validity of this statement is unknown. Autopsies later reveal that Greg is missing vital organs.
On the anniversary of Greg's death, his older brother meets with Chris and Alex to discuss a box of Greg's tapes that he found while searching his room for sentimental belongings. They review the footage and discover that Jesse has been lying to them the entire time. He manages to coax Greg into joining The Order as a means of seeking answers and protection from the mysterious figure following him without notifying anyone, and even lied about not knowing what was happening to Greg in the months prior to his death. When Greg later told Jesse that he intended to leave the organization, the Slender Man arrived at his house and murdered him, with Jesse taking Greg's copy of the Collective Knowledge and fleeing.