Hiyoko's cute appearance, voice, and (initially) her way of talking give off the impression she is innocent and childlike. However, in her introduction alone, it is quickly revealed that she is very direct and cruel with a malicious side in how she squishes ants while telling Hajime that they make a satisfying popping noise if done correctly, and even invites him to join her. Later at the beach, she goes around trying to step on and trample the crabs, laughing as if it were just child's play, showing an alarming fascination with it. She is also described to stomp on and tear out flowers for her own amusement, even though she mentions that she likes pretty things like flowers.

Voice reference: https://www.youtube.com/watch?v=aBuY9MG2a3E&t=153s