The Last Kingdom is fiction. It's a historical fiction series. It takes place in a historical setting, but many of the characters and specific plotlines are made up for the story.
Yes, 'The Last Kingdom' is based on true stories. It is based on Bernard Cornwell's series of novels which are inspired by historical events and figures in the Anglo - Saxon period.
Mildrith is a character in The Last Kingdom novel. She has a complex relationship with the protagonist. She is often caught up in the political and social upheavals of the time in the story.
The Last Kingdom's story is based on real historical events and characters. It shows the chaos and battles as various factions vie for control. The plot is full of intrigue, bravery, and personal growth of the main characters.