These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
There are the Tessa/Will shippers, and the Tessa/Jem shippers, and then there are those who ship Will/Jem.
And then there's Will/Magnus, who are very much canon on Magnus's side.
The Woobie: Will Herondale after we find out the Jerkass attitude is due to a curse that anyone who loves him will die. Even more so when we find out that there really was no curse. The demon tricked him by telling Will he put a curse on him, and then killing Will's sister to make it look like it was the curse.