Kenji was supposed to leave with Noi at the airport!
I never noticed until now....I've just paused the movie to state this.....When Kenji drops Noi off at the airport, Kenji says "Wait for me" Then leaves to get his things, he of course never makes it back.....this is why Noi is sitting in the airport all alone waiting for him at the end but he never shows. I suggest watching the movie again to confirm.
"Wassa mook?"