Honestly I just wanted to draw Zero in a cute outfit and with dog ears+tail ¯\_(ツ)_/¯
The japanese symbols say "wan wan" which basically means "woof".
Despite having a mostly sterile and emotionless impression on people, under the blank face Zero does actually have feelings, opinions and likings, he's just not very good at showing them.
One of those likings would be cute things, mostly anything with animals, but who's to say "No" to an adorable outfit?
( I had to look up japanese school uniforms for this, but like 90% of my search results were just pornsites T-T whyy )