Ninny, the invisible child
Ninny is an invisible child, with a silver bell around her neck to keep track of her. Ninny became invisible after her caretaker, a dreadful lady, mistreated her. The caretaker gave Ninny to Too-ticky who, in turn, took her to the Moomin family.
The Moomins welcome Ninny with open arms. She gets her own room in the attic and already after her first night in the Moomin house her feet are visible. Moominmamma makes Ninny a dress, after which Ninny speaks for the first time since her arrival. As time passes, Ninny becomes more and more visible, until it is just her face missing.
Moomintroll really wants to help Ninny, and tries to teach her how to play. It doesn’t go very well, as Ninny only plays to be polite. According to Little My, Ninny will never become visible again unless she learns how to be angry. This turns out to be true. Ninny doesn’t become visible until she gets really, really angry.