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.
YMMV: Infernal Devices
But Not Too Foreign: Both Will and Jem. They're both half British and half Welsh and Chinese respectively. Will's actual name is Gwilym and Jem's actual name is Jian but their real names are only brought up once or twice in passing and for some reason both seem to prefer to be called by their "British" names.
Wales, alongside England and Scotland are countries within Britain. So Will Herondale is 100% British a more accurate description is that he is half Welsh and half English. As for his name Welsh is very difficult for English people to pronounce it is understandable that when living in London he would use a means of pronunciation that the English Shadowhunters would use.
Ho Yay: 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, 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.