A.S. is introduced to us at the very beginning of the original Room. He is the one who leaves notes throughout The Room and The Room Two. He is friends with the Protagonist, and calls them to his house before the events of The Room.
Though his exact profession is unknown, A.S. seems to be some sort of scientist, researching The Null. He successfully creates a Null Sample, and is warped to the cog room before we arrive at his house.
A.S. once associated with The Circle, but eventually broke off from them, finding that they were "blinded by their dark arts and their greed". He devoted his life to the study of the Null element, continuing his research even after being discredited by his colleagues. His exact methods of research are not certain, but he does make reference to a voyage he and others once took to a hidden temple.
Some of his research was conducted at his Manor House near the Glendon Hills in England. One day, after accidentally exposing himself to the Null, he discovered that his soul had somehow become tied to the element. Later, after constructing lenses from it, he increasingly found himself succumbing to dark visions beyond his comprehension.
In his thirst for knowledge, he attempted to use the energy from the Null to summon the god Astaroth, but the ritual did not go as expected. Bombarded by an increasing number of visions, A.S.'s final note to the Protagonist reports that he is finding more and more rooms in his house.
Some time later, A.S. was transported to the room shown in The Room's Epilogue stage, beginning his long journey across several places in time. The player follows some of this journey in The Room Two, as A.S. attempts to guide them using his notes and the key he constructed from the Null.
A.S. may have used the Null to artificially extend his own lifespan, referencing that he was graced with "time for a dozen lives' work". Eventually, to some end, he allows himself to die so that the player can return home to their own time, possibly using his soul to power a doorway. His body is found in chapter four: The Séance.
“I will try to guide you along the path. I pray you never came. I pray you are just another of my dreams.”
A.S. is a friend of the Protagonist, and often expresses concern for them. He even gives his own life to ensure that they make it back home safely. Their relationship is suggested to have been contentious in the past, with the Protagonist at first resisting A.S.'s calls for their help with his research. Eventually, however, they became too curious and came to investigate.
"The Circle are petty fools; blinded by their dark arts and their greed."
A.S. is known to have some relationship to The Circle, enough that he is familiar with their leadership and their rituals. Whether or not he was ever a member or under their employ remains to be seen. He is shown to have a distaste for their ideology, looking down on their practice of the occult. However, in the height of his mania for the Null, it was a ritual from their Scriptus Nocti that he used to attempt to summon Astaroth.
The Craftsman alludes to knowing A.S., and possibly even trapping A.S.'s soul. Since The Craftsman is not introduced until The Room Three, A.S.'s opinions of him are unknown. However, The Craftsman appears to take a condescending attitude towards him in his correspondence with the player.
Talisman is the company that provided A.S. with the original safe from The Room, as well as the orrery inside and a number of the other boxes. The Craftsman gloats in a letter from The Room Three that "Every soul is unique, and so must be every trap that snares it", and say that for A.S., this was "his box full of demons".
Professor de Montfaucon
In The Room Two - The Lab, A.S. confirms that he has knowledge of Montfaucon's research, even calling it far more advanced than his own. However, he believes that Montfaucon never found the key to traveling through the Null.