One common theme is the struggle between love and duty. Jace has his responsibilities as a Shadowhunter, and the OC often has her own goals or loyalties. Their relationship has to navigate these obstacles.
Another theme is the discovery of self - worth. The OC usually starts out not fully aware of her own capabilities or importance. Through her relationship with Jace and the challenges they face, she grows and realizes her true value. For example, in many fanfics, the OC discovers she has a hidden power or talent that helps in their adventures together.
The theme of acceptance is also prevalent. The OC may be different from the typical Shadowhunter, and Jace has to accept her for who she is. At the same time, the OC has to accept the dangerous and complex world of the Shadowhunters that Jace is a part of. This mutual acceptance forms an important part of their relationship in most of these fanfics.
A practical answer is Chapter 113, because it turns the idea into a concrete scene. The chapter opens with The next day, Jiang Lili was up before dawn as usual., adds pressure through Okay, keep them steady;
If you want the clearest chapter reference, go to Chapter 107. Early on, On the plane, when He Yun had taken off his mask and greeted her...; later, the chapter leans on She had also figured out his current predicament of being trapped by....
Look to Chapter 58 for the clearest answer. It begins with Facing Jiang Yan's triple barrage of questions, Jiang Li's only response..., keeps building through After a long while, she yawned indifferently and said, "I didn't pick up..., and lands on By ten o'clock, Jiang Yan was still wandering.
If you want the clearest chapter reference, go to Chapter 89. Early on, Director Sun took a step back, "What's the rush?; later, the chapter leans on It's true about the go-kart costing one point, but this time we're using....
The best chapter pointer here is Chapter 104, "Hurry up and sign the termination letter, come back to me!". It opens with Zhao Lang let out a light chuckle, "Young Master Jiang, don't be nervous,... and closes on Despite a whole day's labor, his face was sunburned red, his shirt had....
If you want the clearest chapter reference, go to Chapter 112. Early on, These five people are supposed to be at odds.; later, the chapter leans on When did they become so in sync?] [I'm dying of laughter hahaha why is....
Chapter 128 is the strongest chapter-level answer because it starts from Still hungover, Huo Ziyang had just awakened from a blurry dream when his... and follows through on There had been no shortage of Weibo posts criticizing Jiang Li before..
A practical answer is Chapter 84, because it turns the idea into a concrete scene. The chapter opens with When the villagers tasted his curry beef one by one, their faces revealed..., adds pressure through [Pfft, and she's teaching others how to cook, isn't it just scrambled eggs..., and ends on.
Chapter 112 is the strongest chapter-level answer because it starts from These five people are supposed to be at odds. and follows through on [Hahahaha why is the Princess starting to look disappointed?] [Help, even....
A practical answer is Chapter 93, because it turns the idea into a concrete scene. The chapter opens with Jiang Li looked back and saw several aunties linking arms, walking up the..., adds pressure through Even if they were hiring actors, could they really come up with such an..., and ends on I.