When I try to drag to trashcan, it says it's a system app that is required by os.
How do I bypass this check and remove the app?
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There are instructions for removing font book on this page
If you just want it hidden, there's always a
chflags command to hide it. You need to bypass SIP restrictions on that folder before you can delete or change the flags to hide it, so it's not going to be quicker than removing it - just easier to undo if you find you or another program needs it at a later date.
ls -laO / && ls -laO /Applications | grep Font
The first listing shows the variety of flags (hidden, sunlink, restricted, uchg) and the second shows Font Book is restricted from change even by root.
Just like the warning says, Font Book is a system app. That means it's part of the operating system. You can't delete it because it handles important functions that make your computer work. It handles all the fonts on your computer, including the one you're using to read this website. Apple has some info on Font Book in their Mac Basics documentation.
EDIT for the truly determined:
I see what you're saying. However, if you take a look at threads of folks removing Font Book, you'll see all on the older side. Starting with El Capitan, Apple introduced System Integrity Protection. You can disable SIP, but it can be pretty dangerous. However, Apple did make that a tiny bit easier in Sierra by no longer requiring a boot into your Recovery partition to make the change. (That should give you an indication of how strongly this is discouraged!)
Here's a quick article describing the Terminal commands to turn off SIP in Sierra without booting into Recovery. With SIP off, you should be able to delete whatever you want, so you'll probably want to turn it back on as soon as you're done dumping Font Book.