Oshiego to no Doukyo wa Senjou yori mo Ki ga Yasumaranai

Soujurou Toudou is a former mercenary instructor who survived the war with his student, Sylpha. After the war, Soujurou took the orphan Sylpha with him back home to Japan, where he suddenly realized that he had a crush on her. However, he wanted Sylpha to live life as a normal girl, so he decided to step aside and let her go. But to his surprise, Sylpha also confessed that she loved Soujurou, too... A love comedy about a former mercenary instructor and his student living together!

(Source: Flex Comix, translated)