The Green Giant (緑の巨人) is a mystic legendary creature of the Green Planet and is considered as the god of all plants.
In the manga version of the movie, the Green Giant is a god who stopped the wars between plants and animals. He separated the animals and protected the plants from further attack. From then, the plants woshipped him as their protector and savior.
However, Lire also heard a different legend about the Green Giant as a creature of destruction, born to destroy all the animals. Also, the Green Giant possess an ability to transport all plant lives from Earth to the Green Planet.
In the anime version to awake the Green Giant, Lire must first be the one to approve the ceremony. She must also chants the following words:
"The great power of green, please show us your endless power! The anger that breaks apart the land and the sky!"
At first, the ceremony appeared to not work, however, as Kibou is filled with rage and sadness, he began to transform and absorb the giant with the heart of Green Giant and transformed into a large giant tree.
In the manga, only the approval is needed and no chant is required.