Unmarked spoilers are ahead, per Spoilers Off rule.
Sawako's appearance, particularly when forcing a smile, was Nightmare Fuel to other people for the first fifteen years of her life. It's even worse when she's soaking wet from the rain.
2nd Season, Episode 3: Sawako gives out the scariest unsmile she has ever made.
At the cultural festival parade, Sawako's acting is so convincingly scary that it even freaked out the judges.
The flashback to Ryu's mother's death in Chapter 63 is surprisingly scary, but in a different sort of way.