Sizzy

Sizzy is the het ship between Simon Lewis and Isabelle Lightwood from the Shadowhunters fandom.

City of Bones
When Simon first met Isabelle, he was in love with Clary Fairchild and while he was attracted to Isabelle, he only saw her as a way to make Clary jealous. Through Clary, the two became constants in each others lives and friends.

City of Ashes
The two slowly become closer once Simon becomes a vampire.

City of Glass
Simon was able to comfort Isabelle after the death of her brother, Max. After that, their relationship blossomed. They began dating after the war, but Simon was also dating Maia Roberts at the same time as he was under the impression that they weren't exclusive. He didn't tell either girl, and both Izzy and Maia found out. Furious, they both broke up with him. Isabelle and Simon made up fairly quickly after this, but they decided to remain friends.

City of Fallen Angels
Their connection further deepened as they spent more time together. Their feelings for each other became undeniable, however, neither one of them were willing to open up due to the possibility of heartbreak. Though they were essentially together, the pair refused to admit it. This continued over the next book.

City of Lost Souls
Though Isabelle and Simon were essentially together, they refused to admit it. They had trouble defining their relationship.

City of Heavenly Fire
At the beginning of the book, Izzy and Simon had trouble defining their relationship, however, they both ended up admitting their feelings for each other in Edom. Simon ended up sacrificing himself - his immortality and memories of the Shadow World - for their safe return to their world. Isabelle was heartbroken but gained hope when Simon decided to try to Ascend to regain his memories.

Tales from the Shadowhunter Academy
During his path to regaining his memories, Simon tried to keep his distance from Isabelle and Clary, conflicted about the feelings they had for his old self - the one he couldn't remember - and intimidated by the memories they shared but he couldn't recall. Eventually, he came to accept that they cared for him regardless of his memories, and he and Isabelle made an effort to get to know each other again. They began to date during their free time, and while it was awkward and difficult at first, they worked it out. They slept together for the first time the night before his Ascension ceremony.

Lady Midnight
In 2012, Simon spontaneously proposed to Isabelle, who accepted.