Se-Young is a foodie and loves to eat so they call her Miss Piggy.
Kim Young Kwang is a playboy who likes Se-Young.
They have a "past" thingy but act like it never happened and I don't think it shows the flashback part of it until like, episode six or somewhere around there.
This couple dated, I think they were in college at the time. I'm going to say what happened when they broke up because it's what sets the whole base for their story so it's not a spoiler at all. Kwang Soo proposed to Da In and she rejected him by hitting him on the head with the bouquet flowers that he gave her. It sounds really mean and rude but you'll eventually figure out why she did it - though it does take a while to get there.
I had no idea that Soo Ah was played by Chorong at all! I didn't recognize her and I didn't even know she was in A Pink. I just thought she looked familiar until the other day when I had Mr. Chu
I'm not big on their story line but maybe it's because they show the other three couples more. I don't know.