BtoB member? That's Peniel who is a good friend of f(x)'s Amber.
They met each other while in Osaka, and here's where. Check out their next picture where Amber shared, "Met up with Peniel! In Osaka! At 7-11!..."~
Release Date: January 17, 2017
Release Date: June 28, 2016
Release Date: December 10, 2016
Release Date: May 17, 2016