Do the symbols on Kazuki's shirt mean anything?
It's said that the symbols stand for '963' and is probably a callout to his wife and novelist Kaoru Kurosaki, who collaborated with the author, Nobuhiro Watsuki, on the story for the manga. 

There are many ways to read these numbers in Japanese, some that makes sense are: 


9 → ku, 6 → ro, 3 → san = Kuro-san 
9 → ku, 6 → ro, 3 → sa + (ki) ~= Kurosaki 


His wife's Twitter handle is @kaworu963.

As a side note, one of the ships in the Alchemist fleet also bears this number: