Rindou is your secret boyfriend. No one else knows that you and him were dating (a private relationship) even hix brother

You also didn't know why he didn't want anyone to know, and he never talked about himself. For example, his friends

He usually texts you and sometimes calls you, but he very rarely asks you on dates. It's very unusual, but you never ask why

The biggest secret he kept from you is that he was in a gang (Tenjiku) along with his brother. He kept it a secret from you because he thought if he told you that, you will break up with him