Hela, goddess of the underworld in charge of the souls that don't make it to Valhalla. However, she isn't the cause of Ragnarok, but makes up a small section of Ragnarok.
Hela's actual family, Loki, who is her father, Fenrir, her brother, who is the giant wolf in the movie , "Thor: Ragnarok." And the sea serpent, Jormungandr, who can wrap himself around the world and bite his own tail.
Loki and Heimdall kill each other. In ancient Norse Mythology, Loki and Heimdall are enemies. The rivalry comes to a head in Ragnarok when Heimdall kills Loki, but Loki also kills Heimdall as well.
Odin and the Valkyries or the women who reaped the souls of warriors fallen in battle for Odin's own army of war-dead in Valhalla.
Surtr the fire giant will lead the mysterious sons of Muspell to battle in Ragnarok. He will come with his flaming sword, and will consume he entire Earth and world.
Thor and the giant serpent battle it off, but even though he was able to kill the serpent, it doesn't take Thor very to be dead by the poison of the serpent. Both the serpent and Thor die in the end.