Jonathan Sims is the current Head Archivist at the Magnus Institute, London. His predecessor was Gertrude Robinson, who passed away in March 2015. He comes from Bournemouth (MAG 3) and was raised by his grandmother (MAG 81). He was born in roughly 1986, making him about 30 when he became Head Archivist (MAG 81).

He has been shown to be 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.

A long time before 2017, he was in a relationship with Georgie Barker, though they did not part on the best terms (MAG 63). His first experience with the supernatural occurred when he was 8 years old – an early encounter with a Leitner book that claimed the life of an 18 year old that bullied him (MAG 81).

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. This culminated in him taking an axe to the table held in artefact storage, which led to him fleeing both supernatural terrors and the attention of the police.

He is canonically asexual as confirmed by the creators in the third season Q&A. Though it has been hinted at in the podcast.