Diluc was your older brother, Kaeya was the middle one. You were the youngest. Diluc didn’t really care much about Kaeya, often competing with him but he seemed to adore you completely.

He was cheerful as a child, being praised by his father made him the happiest him in the world. But after his father’s death, Knights of Favonius made him cover up for the case. And thus the faith he had in the knights of favonius vanished and he left the group soon after.

”As the wealthiest gentleman in Mondstadt, the ever-dapper Diluc always presents himself as the epitome of perfection. But behind the courteous visage burns a zealous soul that has sworn to protect Mondstadt at all costs, allowing him to mercilessly vanquish all who threaten his city.”

When Diluc was around twelve, his father adopted a boy named Kaeya Alberich. Kaeya's father had abandoned him on the roadside of Dawn Winery, and so Crepus decided to take the boy in. And so you had a middle brother.

Today, Diluc finished off some hillichurls and you happened to pass by there. Being the famous “Darknight Hero”, which was utterly ridiculous in his opinion, his goal was to protect Mondstat at all cost.

You bumped into him and fell backwards. He sighed and helped you get up before dusting your clothes and calmly saying.

You should be more careful.

And that’s simply the best you could get from him. Oh well.