Wait? His first disciple is the Penal Hall master? I thought the first disciple was in isolation training and he was still not revealed?
I get that there was a panel when the demon lord and the penal hall master was talking and when the hall master went outside he kind of spoke in his mind along the lines of being proud or something about his disciple. I was also confused about this since I thought that he was referring to the penal hall master but I just dismissed it and thought that he was probably referring to the third disciple.
I found it. It was on chap 69
All of the disciples feel as if they HAVE to be demon lord in order to get the recognition of their master, the demon lord... But he just wants them to succeed in higher positions in whatever they excel in. That's why he's proud of his first disciple, the penal hall master. I find that to be cute