A Null user can be defined as an individual who interacts with the Null element, willingly or otherwise. There are many characters throughout the series that are interested in solving the mysteries of the Null and are known to interact with it to some degree.
A.S. is the first character to explain the properties of the Null to the player. He presumably began researching under the guidance of The Circle, but broke away from them to continue on his own. He saw the Null as the element between earth, air, fire, and water. He distilled his first sample of the Null using his own soul, which eventually caused him to fall to the Null's influence. Eventually, after being trapped for many years traveling through room after room, he discovered that souls could be used to manipulate the direction that the Null took him in. In The Room Two, he sacrifices his soul to send the player back to their own time and place.
The protagonist of the main trilogy was first exposed to the Null through A.S.'s research. After answering a letter asking them to come to A.S.'s manor, the protagonist begins their own journey through the passageways created by the Null. In the first game, they know very little about it, but by the events of The Room Three, they have done certain research of their own, as shown by the notes in their journal. They also feel an insatiable curiosity towards the Null that only vanishes if the player achieves the Escape ending.
The Priestess (or "Baia", as some have speculated) is a minor character from The Room Two - The Temple. She attempted to use the Null to restore power to her struggling civilization, and saw visions of the past and the future. Something she saw prompted her to give up the use of the Null and seal it away in her temple so that no one else could use it.
Professor de Montfaucon
Professor de Montfaucon was a scientist studying the Null in The Laboratory of The Room Two. His primary area of research was the way in which the Null could be used to restore a semblance of life to dead organisms. Using the Null, he applied energy to a human arm and a human heart, causing them to move and beat as they might have in life. As a result of his research, he reached the conclusion that his was not the only sample of the Null in existence, and that there was some network of interconnected samples at play. Despite his progress, A.S. believes that Montfaucon never found the "key" to the Null.
The Craftsman is a powerful Null user, capable of crafting machines to harness and gain some semblance of control over it. He is adept at using the Null to trap and harness the souls of many victims, which he uses to power his contraptions. He especially targets other characters who take an interest in studying and using the Null. His main objective is to reach the Null planet, which he believes is the source of the Null. In A Dark Matter, The Witch says that The Craftsman has served many masters over the years, and that the Null is only one of them. Despite this, he regards the Null dimension as his domain - perhaps foolhardily, as it is the Null that eventually consumes him and collects its "debt".
Collector Hydrus is a collector employed by The Circle to retrieve Null artifacts. He is able to navigate the Waldegrave Manor dollhouse and break the seal keeping the Null sample locked inside. The Circle leadership disagrees with his methods of collection somehow, considering them to be "vulgar". Though he does not "use" the Null himself in The Room: Old Sins, he seems to have his own designs for the Null that would go against the wishes of The Circle, and is biding his time to act on them.
The Witch is a Null user of abnormal power. She exhibits a particular mastery over the Null, even more so than The Craftsman. One of the doors in her house gives access to the dimension of the Null, where she can summon any room of her choosing using Tarot Cards. She is also able to manipulate her soul outside of her body with considerable ease. At the end of The Room VR: A Dark Matter, she and The Detective work together to use the Null to bring down The Craftsman.
Rigby was a grave robber hired to find a Null artifact in a crypt. Judging from some of the markings left on a letter from his employer, it would appear that Rigby became interested in taking the Null sample he was hired to find for himself. Rigby's journey is mostly unknown, though the voice of the typewriter in The Room Two - The Séance suggests he is one of the many travelers through the Null in an Easter egg.
Margaret Cox was a fraudulent fortune teller and spirit medium. Her exact connection to the Null is not well-defined, though she uses a particularly ancient Tarot deck which includes "The Null" as one of its cards. She used the Null as a part of her act, along with Simon Grayson. Maggy was also an acquaintance of The Craftsman, who eventually used the Null to imprison her. Their former relationship is unknown, but Maggy appears to have some connection to Grey Holm, as paintings of the manor can be seen in her apartment.
Simon Grayson, a.k.a. The Great Khan, was a magician, and the mentor of Margaret Cox. He used a Null shard to create illusions as a part of his magic act, and is even featured with it on some of his promotional materials. The Craftsman eventually had him imprisoned at Grey Holm as punishment for his knowledge of the Null.
Edward Lockwood was a chemist hired by The Circle to research the Null. His research gradually led him astray and began corrupting his home until his wife, Abigail, stepped in to try to stop him. When Abigail stole his Null sample, Edward was driven by madness to pursue her throughout their manor until the Null was imprisoned and its influence wore off on him. At this point, he gave up his search and accepted his fate. His soul was trapped in the sample found at the end of the game.
Abigail Lockwood learned of the Null by secretly studying her husband while he researched. Though she possessed the intelligence to replicate Edward's work, like him, she lacked the understanding of the Null to control it. When she stole the sample and attempted to flee, it took all of her willpower to seal the Null away in order to escape.
The Detective first learns of the Null through The Craftsman, who offers to help them locate Dr. Rupert Montgomery, a missing person. With The Craftsman's guidance, they learn to use the eyepiece, and are able to find the souls of several Null users trapped within artifacts. Later, The Witch steps in to keep The Detective from falling into The Craftsman's trap, and the two use the Null against him to defeat and imprison him. The Witch expresses the hope that The Detective will continue to use their newfound knowledge of the Null to hinder those who would use it for their own nefarious purposes.
Dr. Rupert Montgomery
Dr. Montgomery was an Egyptologist and one of The Craftsman's victims. The Craftsman, disguised as a porter, introduced Montgomery to the mysteries of the Null, and used his curiosity against him to imprison his soul in an artifact.
The Reverend was interested in finding the location of an ancient Null relic once held by the Knights Templar. The Craftsman took advantage of The Reverend's fervor to lead him down the path to the Null and trap his soul.
Based on Easter eggs found during The Séance, some suspect that the privateer captain of the Rose from The Room Two, tentatively called "Hirst", may have interacted with the Null in some way. He was introduced to the Null after pursuing a ship known as the Santa Helena, which wrecked off the coast of South America. After rescuing the wreck's sole survivor, the captain discovered that he was carrying some sort of artifact and took possession of it. Because of the presence of Null on board, the Rose later suffered the same fate as the Helena.
The Circle are a religious organization devoted to the Null. They favor faith over science, and somewhat worship the Null. They are known to send collectors to retrieve Null samples and artifacts, which they use for their own rituals. They connect the Null to their own practice of demonology.
The Royal Institute
The Royal Institute is a broad scientific institution. They are said to have knowledge of the Null, but deliberately deny and refuse Null research. When the protagonist approaches them about A.S.'s disappearance and their findings on the Null, the Institute turns them away.