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.
Expy: Will, who is basically Jace with different coloring. It's worth noting that Jace is an expy of the original Draco in Leather Pants. Critics argue that the rest of the cast are Expies as well.
It's true. Jem is a straight Alec (or in the case of the love triangle, Simon). Jessamine is a pre-feminism Isabelle. And Tessa is Clary with an attitude.
YMMV. Once analyzed, the only thing Will and Jace share is their snarkiness. And their last name
YMMV for ALL of the characters mentioned, considering that all Jem and Alec share is a parabatai connection with a Herondale and the only similarity with Simon is being part of a love triangle though not as quickly disposed of as Simon was in his love triangel. The only thing Jessamine and Isabelle have in common is being pretty, especially with Jessamine rejecting everything about being a Shadowhunter and being shallow and selfish for most of the series, and Isabelle loving her heritage and while enjoying fashion and make up, tends to put those she cares for above all else rather than fretting over her looks in times of crisis. Tessa and Clary, much like Jem and Alec, only really have superficial similarities, such as their Herondale boys, in common, as well as having complicated family situations and discovering they are different from what they were raised to believe. On a deeper level, they are incredibly different from one another.
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.