"Zeno-sensei!" And that was the death of Kiriga respectfully calling him 'Ouryuu-sama' in 1880.
Kagami looks more than absolutely joyful. Like nothing could ruin his own magnificent realization. Kiriga, 19-years-old and not able to keep a straight face as chuckles escape the man at his 5-year-old son's enthusiasm.
Zeno looks at Kagami on Kiriga's lap, who had been for a while now wondering just what exactly to call the great Ouryuu-sama. Because Zeno was too informal and 'Ouryuu' was not a cute nickname. Then the 5-year-old had blurted out Zeno-sensei after hearing about how Zeno helped Kiriga with his studies.
Since Zeno 'even helped his great and amazing father', as Kagami put it, 'and as he was now teaching Kagami' Zeno should be called a teacher apparently.
And then Kiriga, the brat went and also adopted the nickname. And since then he had not heard even one Ubuyashiki call him anything but Zeno-sensei. Even the sturdy women of the house, specifically Kiriga's mother Akari had taken a liking to it.
The nickname though. Zeno savored it. No one had given him one since Hinoe-