lemonade heart
someone who's usually miserable, but lots of people are nice to them, resulting in them slowly learning to be happy. lemonade is sour without sugar, but once sugar is added it's enjoyable.
Person A: Man, insert name here is always miserable, but everyone being so nice seems to make them happy."
Person B: "Yep, classic lemonade heart."
Person B: "Yep, classic lemonade heart."