Lee Ji-hyun as Romi (voice)

Jo Kyoung-i as Heartsping (voice)

Choi Nak-yoon as Heart King / Monster Truthping (voice)

Yeo Min-jeong as Belita / Girl (voice)

Hong So-young as Stella (voice)

Hong Bum-ki as Dr. Monziu / General Teenieping (voice)

Kim Ha-yeong as Marie (voice)

Ryu Seung-gone as Liam / Butler (voice)

Cho Hyun-jung as Truthping (voice)

Kim Eun-ah as Happying / Young Liam / Girl's Mom (voice)

Jang Ye-na as Lalaping / Maid / Stickping (voice)

Um Sang-hyun as Mr. Bus Stop / Freezeping (voice)