Simon is a gangling young man with eyes like two watery poached eggs. He suffers from terrible hay fever and an even worse stammer. He can't pronounce 'w's and begins to panic when he spots one in a sentence he is trying to say. Fortunately for him, people invariably supply the word themselves, but the gratified look of thanks he gives is embarrassing to all. He teaches himself magic out of old books and becomes so powerful that he comes to the attention of those at Unseen University. Treatle, an eighth-level Magus and then the second most powerful man on the faculty is sent to get him and bring him to the university to study. On their journey to Ankh-Morpork they meet Eskarina Smith who wants to become a wizard. At Unseen University, Simon immerses himself in his studies, reading vigorously and even giving lectures (unheard of for first year students!) to which all the senior wizards come. They are stunned to find that he has created and understands truths of which they have only the haziest notions; thaumatological truths as to the nature of magic and its uses, the whys and hows of it.
His powers inevitably causes ripples in the space-time continuum and the "Things from the Dungeon Dimensions" cluster in his shadows with their characteristic chittering noises unnoticed by all except Esk with her own burgeoning magical talent. Eventually they burst through into the Discworld Dimension and take Simon with them to the Dungeon Dimensions, whence Esk follows to try to rescue him. She manages this by not using magic, which puzzled and infuriated the Things who are thaumivores - they feed on magic. When she and Simon eventually returned to Unseen University they set up a research group and come up with an entire system based on the non-use of magic.
there is a theory that Simon and Esk develop a relationship and have a son but there is no real hard evidence to this beyond the fact that Esk's son is mentioned in passing and she is never mentioned as being with anyone else but Simon. Simon's current status is unknown. He is presumed dead - killed in the collapse of the Tower of Art during the battle with the Sourcerer - as no mention of him is made in any subsequent books (he is mentioned in I Shall Wear Midnight, but in the past tense).
In I Shall Wear Midnight, Simon is described as having let his illnesses and allergies multiply to the point where he has become Unseen University's analogy of Stephen Hawking - speaking to peers through a machine (Hex?) and unable to move, talk, or do very much for himself. This is related via Esk, who can be described as a reliable witness.
..."the young Eskarina had met at the University a young man called Simon who...had been cursed by the Gods with almost every possible ailment that mankind was prone to. But because the Gods have a sense of humour, even though it's a rather strange one, they had granted him the power to understand - well - everything. He could barely walk without assistance, but was so brilliant that he managed to keep the whole universe in his head. Wizards...would flock to hear him talk about space and time and magic as if they were all part of the same thing. And young Eskarina had fed him and cleaned him and helped him get about and learned from him - well - everything." (I Shall Wear Midnight,
Simon describes part of the knowledge as elasticated string theory, a phenomenon which Eskarina says, in a discourse with Tiffany, has at least sixteen different dimensions.