Nico

Nico lives in a fairytale picture book that Miyuki picked up from a cart of picture books when she was younger. Being curious, Miyuki began to read it. Nico's words in the book helped her to understand "The key to becoming friends is to smile", and allowed Miyuki to find some friends. Miyuki promised to finish the book, which the last few pages were tore off. She drew a lot of pictures, but Miyuki lacked drawing skills. Eventually, she gave up and lost memory of it.

At first Nico was angry at Miyuki, feeling betrayed by her after years of being left alone after Miyuki promised her to draw a happy ending for her. She willingly befriended the Demon King in order to seek revenge. She, however, soon learns that she feels no better from this and Miyuki helps her feel loved again, allowing her to let go of her grudge and return to her once happy, always-smiling personality.

(Source: Pretty Cure Wiki)