The Mortal Instruments
Jace originally despised and looked down upon Simon. However, Jace did allow Simon to suck his blood while Simon was a vampire. Jace even admitted that he liked it and found it rather homoerotic. They eventually became friends/quest buddies.
OF MEN AND ANGELS
Jace yells at for the mundane to keep up, which Simon wishes he would stop calling him. Jace doesn't want to learn his name, since he doesn't care. Simon says that means he would have to care about someone other thank himself, however Jace does care about other people, just not Simon. Simon says that he probably doesn't care about Alec either, which Jace take offense too. Simon says that Jace treats Alec like a pet, and Jace calls him Clary's. Simon says that Clary is his best friend, and Jace tells him that's all they'll be. They start to argue over Clary and Alec, before Simon angrily pushes Jace up against a wall, holding a knife to his throat.
Although the two were shipped, it began to take off more during the second book. Especially after Simon drank Jace's blood in order to live. Fans of the books were disappointed that the scene wasn't adapted as faithfully in the tv series.
On AO3, Jimon is the third most written ship for Simon and the second most written for Jace. It is also the fifth most written ship in the Shadowhunters (TV), The Shadowhunters Chronicles - All Media Types and The Mortal Instruments - Casandra Clare tags, and the seventh most written in The Mortal Instruments (Movies) tag.