Portrait Values can be changed by entering 6 hex byte values in.
A hex value is two characters from 0-9 and A-F
i.e. 0F is a byte, 15 is a byte, etc. It’s all in base 16, which is a bit difficult to wrap your head around if you don’t know how base conversions work.
For example, if you see DEDFE0E1E2E3 (Cao Zhen’s avatar) try changing it to DEDFE0E1E2E7 (change the last byte) and you’ll notice the bottom part changes when you save.
Enemy AC is the defense of that general when you fight them. An enemy with high AC will be really tough to damage.