This is the relationship between adopted siblings Klaus and Viktor Hargreeves.
Klaus was drunk when Viktor arrived at the Academy. Sir Reginald died and he was talking to Luther about him. He was making a joke for drinking his water. The two often did not interact, as Klaus was often more interested in getting high and generally took nothing seriously. However, after discovering that Luther locked Viktor in the vault, Klaus pleads for him to be let out, citing that he was obviously struggling with the discovery of his new powers. In the second season, Klaus is shown to be very soft towards Viktor. They start to communicate more often. Him, Allison, and Viktor become closer and reunite. Due to his memory loss, Viktor doesn't quite remember Klaus, but nevertheless, they grow closer and form a new bond with one another. In episode 10, he's the first one to get in the car with Viktor on his trip to Sissy's farm, citing that he "can't let [Viktor] face the unknown alone." Klaus and along Diego and Five were accepted Viktor.