What is a word to describe when someone is upset that someone else thinks little of themself?
Solution 1:
If you really can't think of anything, imagine their expression and describe that (maybe their eyebrows are doing something, or their eyes are bright or sad or their mouth is in a line, ect.)
This might not be helpful, but:
Sympathetic
Offended
Pained
Whistful
Indignant is a good one though (you could say "Character A gave B a look, indignant on his/her behalf. 'Yes, you.'")