During the "Mister Mister" part, the song writer "The Chikk" admitted in an interview that it was her voice mixed with SNSD. I think she is the one you are looking for when you don't know who is in the back
I think Mr Mr chorus lead vocal is supposed to be Jessica. You can clearly hear her voice throughout entire the chorus. U can try filter jessica's lead vocal
WTF Dance Line singing together in the bridge? 😮❤ So the analysis like this right? 👇 𝐋𝐄𝐀𝐃 Sooyoung+Yoona (𝐁𝐀𝐂𝐊𝐆𝐑𝐎𝐔𝐍𝐃 Hyoyeon+Yuri) + Taeyeon ad-libs 𝐋𝐄𝐀𝐃 Hyoyeon+Yuri (𝐁𝐀𝐂𝐊𝐆𝐑𝐎𝐔𝐍𝐃 Sooyoung+Yoona) + Tiffany ad-libs