Responses
he has memories of them as children and the title alludes more to him posing as his brother when giving out revenge. Thats why he said at the start that he would be entering the tower as his brother
They look alike because they are twin. Cain have his brother diary and he's living as his brother to get revenge.
Spoiler
Cain and his brother will meet each other some time later. His brother's soul is in the pocket watch
tell me am i the only one who thinks that cain and his brother are one? like the face and voice are the same, and the title 'second time ranker' isn't much of a coincidence right? spoil meee