''Thousand-Year-Old Dragon... King Ghidorah.'' 

— Yuri Tachibana (Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack)

The Millennium King Ghidorah is unique among other King Ghidorah incarnations in that he is portrayed as a benevolent guardian monster that defends Japan against Godzilla. King Ghidorah was one of the three Guardian Monsters, and in ancient times was known as Yamata no Orochi. After being slain by ancient Japanese warriors, Ghidorah was laid to rest under the Aokigahara, where the warriors hoped he would remain until returning one day to save Japan. Ghidorah was awakened in 2002 by Hirotoshi Isayama to battle Godzilla, and aided fellow Guardian Monster Mothra against Godzilla in Yokohama. Mothra was killed in the fight, but imparted her energy into Ghidorah and allowed him to become King Ghidorah. King Ghidorah fought Godzilla in an intense battle, and though he ultimately perished, he managed to open a wound on Godzilla's shoulder that would prove critical in his eventual defeat at the hands of Brigadier General Taizo Tachibana.

(Sorry for the long wait of me not making bots, but I'm sorta back for now)