As the Archivist, he is able to compel people to answer questions he asks, and have them give well-told statements about their supernatural experiences. He is also developing the power to know things he shouldn't otherwise be able to, sometimes without realizing it.
Jon is canonically asexual as confirmed by the creators in the third season Q&A, though it has been hinted at in the podcast (MAG 106).
Jon was a smart but difficult child. He got bored easily and would wander off. His grandmother found books were helpful for keeping him in one place, but he hated to read anything he felt like he'd already read before, meaning most authors could only provide him one book.
His first experience with the supernatural occurred when he was 8 years old, around 1995/96 – an early encounter with a Web-related Leitner book that claimed the life of an 18 year old that bullied him (MAG 81).
He took a job with the Magnus Institute's research department in 2011, four years before being made Head Archivist in 2015 (MAG 1).
Season One Edit
Jon starts out cranky at Gertrude and dismissive of many of the cases, believing that reports of the truly arcane or esoteric are few and far between, though events of late are starting to seem worrisome. He has done almost all of the audio recordings for the Magnus Archives. He is helped by researchers Martin Blackwood, Sasha James, and Tim Stoker, though he seems to hold Martin in particularly low regard, at least until after the latter’s encounter with Jane Prentiss (MAG 22). Jonathan Sims is keen to digitize the Archives; however, attempts to record certain cases onto his laptop have caused significant audio distortions, so he has had to settle for recording onto archaic tape recorder.
Following Martin's encounter with Jane Prentiss and her subsequent siege of the Institute (MAG 39), he has become more and more concerned about the dangers of his job, including a sense that he and his co-workers are being hunted.
Jon survives the Prentiss attack with worm scars and significant mental trauma, stalking and mistrusting his assistants.
Season Two Edit
Jon stalks his assistants at their homes and spends a significant amount of time after work exploring the tunnels. He gets Basira Hussain to bring him tapes recorded by Gertrude from her murder investigation.
Jon eventually takes an axe to the table held in artefact storage, which leads to him fleeing supernatural terrors. He is then rescued by Jurgen Leitner, who tells him about the existence of The Entities before being murdered by Elias Bouchard.
Season Three Edit
Wanted for Leitner's murder, Jon hides out at Georgie's house and gets Melanie to find info for him from the Archives while he's on the run.
He goes to Jude Perry and Michael Crew for statements. As Michael Crew finishes his, Alice "Daisy" Tonner knocks on the door and tries to kill Jon. Basira saves him, and they go interrogate Elias about Gertrude's murder.
Jon and Daisy later capture and interrogate Sarah Baldwin about The Unknowing, which gets the attention of Nikola Orsinov, who first tries to get him to find the ancient skin Gertrude stole, but later just kidnaps him, intending to use his skin for the ritual herself. Michael shows up to kill him before she can, but is replaced by Helen, who decides to rescue Jon instead.
Jon stops The Unknowing with the help of his assistants but ends up in a coma.
Season Four Edit
Jon wakes up from his coma after a visit from Oliver Banks and subsequently starts referring to himself as "The Archivist."