Considering that Kagami was just a plot device to show that Tobirama is a good person (which didn't help at all lmao) I would like to express a couple of my headcanons to add some personality to this guy.
Kagami was the first to breed crows in the Uchiha clan, he loved to make birdhouses and feeders for them for the winter;
Kagami was good friends with Torifu Akimichi, and got along better with him than anyone in the escort squad. They had been best friends since the academy;
Torifu and Kagami were called two sheep at the academy, due to Torifu's weight and Kagami's curls. Hiruzen stood up for them, and this deceived Kagami's perception for years into believing Sarutobi to be a good person;
Torifu was fed to Kagami's crows when he died;
Kagami often separated Hiruzen and Danzo when they fought and complained about them to Tobirama. They didn't have a very good relationship;
Kagami was Izuna's ancestor, but from his youngest son, so he is quite far from the place of the head of the clan;
Sometimes Tobirama took Kagami fishing, but Kagami liked fishing alone more than with others (not out of personal dislike, he respected Tobirama, he just liked silence more);
Kagami specialized in genjutsu;
Kagami's favorite food was nikujaga (vegetable stew with chicken)
Kagami's favorite genre of books is fantasy;
He wanted to become Hokage, but realizing that this was not a very nice cause, he decided to better become a jounin sensei;
Kagami wants to become a jounin sensei when he turns 30, but he doesn't have time;
Kagami's wife was a chronicler;
Kagami's child's name was Kotori;
Kagami was afraid of dogs and therefore did not make contact with the Inuzuka;
Kagami doesn't eat eggs because his crows are side eyes him.
