I think it means sei knew about kaoru watching them and helping Kanao, so im assuming that sei used kaoru like a double agent made kaoru tell him what Kanao was doin.
But Kaoru is kanao's brother, not the stalker... The stalker is Konno (⊙…⊙ )
I'm soooo confused...
Sei already knew that the neighbor was watching them all the time
Kanao made a deal with neighbor Konno, Sei made a deal with Kanao's brother Kaoru to get the neighbour out of the way, that's him texting Sei. Sei knew that Konno was still watching the couple do the devil's tango so he said "it's nice to be alone" since Konno couldn't watch them when they were on a vacation. They're both manipulating third parties to keep their significant other by their sides, but in fuvked up ways.
The deal between Sei and Kouru was so subtle... Loving this twisted shit.
"it's nice to be alone"
Can someone explain this part to me? (╯°Д °)╯╧╧