No one:
Taeil: "Happy Christmas~"
Haechan: "It's Merry Christmas."
Merry Christmas Correction
Explanation
Explain Bear
Listen up, dummy. You probably think Kpop idols just dance and sing, huh? Well, Haechan is correcting Taeil's holiday greeting. "Happy Christmas" ain't technically wrong, but "Merry Christmas" is the more common way to say it. It's a playful jab at Taeil for not keeping it traditional.