Responses
It's basic writing tho, really.
Given that the characters are never shown having a chit chat about life and that Mugi phrased it that suggests he KNOWS rather than he has been told makes it easy to think he's some mental nutcase that stalked the black haired guy or looked into him after he showed up at his shop.
Admittedly, it could be an error of translation too but with all the freaky vibes and 'psycho pannels' (as I like to call them) we get to see of Mugi it's really not such a far fetched idea to think he's stalked him or looked up his past somehow. X'D
Yall saying how Mugi knows Rokus mom when mangas time skip. So chill with your titties cause you dont rlly have to know EVERYTHING about there life.