Devil's are beautiful to fool you if you hadn't noticed you've been fooled XD but seriously I just think the poor guy has trauma and he's inflicting it onto someone who's weaker than him and I think that is what the old guy was saying to him basically
Yes sarah had some relations with the dead friend. Yeah sarah blamed ren for it but in the previous chap, i think sarah forgives ren and wanted to relay it thats why she's visiting japan. My theory is once ren is forgiven and removed his burden,i bet he would reveal that he is actually kuon.
I believe the name of the gf of Ren's friend is Tina. And her mother name is Juli, and her hair is blonde.But maybe she's really Tina or Juli in disguise. Hime Hoshina
reading this is like being slowly killed and the murderer is using a spoon