I think that's Big Sejin you're referring to. I remember them calling him Big because he's older than the other Sejin. But my head's a mess so feel free to correct me please
big sejin is the same age group with moondae and ahyeon... lee sejin A is the actor/the one who collapsed
Big Sejin is Lee Sejin B, the same age line as Moondae and Ahyeon. Novel reader call him Keun Sejin. The other Sejin is Lee Sejin A, small/tiny sejin, the one who collapsed. He's the oldest member in the group.
And novel spoiler ~
Small Sejin or Sejin A will change his name from Lee Sejin to Bae Sejin later in the story.
At this point l don't care so much for the other members except tiny sejin. Like the hate he gets is so undeserved and he even collapses!!!! I hope he gets better and those haters go to hell