The Moon Pavilion was a work of Xu Youzhen in the Ming Dynasty. It described the windows upstairs and the scenery of the River Moon Pavilion. The specific poem is: "Zheng Jun's family lives in Baihua Island, and the tall building is a hundred feet high." Come out of the sea at the beginning of the bright moon in the evening. Let's shine on the top floor of the Jun residence first." These poems depicted the scenery of River Moon Tower under the bright moonlight and the windows upstairs.
One key element could be his origin as a human. His human experiences surely had an impact on his demon self. Another might be any significant relationships he had before becoming a demon.
Upper Moon 4's back story could involve a tragic event in his human existence. Maybe he was betrayed by someone he trusted deeply. This betrayal could have been so severe that it pushed him over the edge, and when he was turned into a demon, his new nature was shaped by that past hurt and anger. His actions as an upper - moon demon are likely influenced by the unresolved emotions from his human days.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.