Who is Totoro in the 1988 Japanese animated fantasy film "My Neighbor Totoro"?

My Neighbor Totoro is a 1988 Japanese animated fantasy film written and directed by Hayao Miyazaki and animated by Studio Ghibli for Tokuma Shoten. The film—which stars the voice actors Noriko Hidaka, Chika Sakamoto, and Hitoshi Takagi—tells the story of the two young daughters (Satsuki and Mei) of a professor and their interactions with friendly wood spirits in postwar rural Japan. The film won the Animage Anime Grand Prix prize and the Mainichi Film Award and Kinema Junpo Award for Best Film in 1988. It also received the Special Award at the Blue Ribbon Awards in the same year.

In 1958 Japan, university professor Tatsuo Kusakabe and his two daughters, Satsuki and Mei, move into an old house to be closer to the hospital where the girls' mother, Yasuko, is recovering from a long-term illness. The house is inhabited by tiny creatures called susuwatari—small, dark, dust-like house spirits seen when moving from light to dark places. When the girls become comfortable in their new house, the soot spirits leave to find another empty house.

One day, Mei discovers two small spirits who lead her into the hollow of a large camphor tree. She befriends a larger spirit, which identifies itself by a series of roars that she interprets as "Totoro". She falls asleep atop Totoro, but when Satsuki finds her, she is on the ground. Despite many attempts, Mei is unable to show her family Totoro's tree. Tatsuo comforts her by telling her that Totoro will reveal himself when he wants to.