The main characters in 'Pen Pal' are a fascinating bunch that keep you hooked. There's Alex, the introverted writer who starts the pen pal exchange, pouring his heart into letters while hiding his real-life struggles. Then we have Sam, the mysterious correspondent who seems to know way too much about Alex's life. The chemistry between them builds slowly through their letters, with Sam dropping cryptic hints about their connection. There's also Maggie, Alex's best friend who suspects something's off about this pen pal situation, and Detective Chen who gets dragged into the mystery when things take a dark turn. The way these characters intertwine through letters and real-life encounters creates this eerie, compelling dynamic that makes the story impossible to put down.