Due to her anecdotes and vestiges, she is classified as an "anti-hero."

Assassin's True Name is Shuten Douji, one of Japan's Three Great Monsters.

In the Heian Era, Shuten was the king of Mount Ooe was a commander of many fearsome demons.

There are many origin stories surrounding Shuten Douji. One goes that she was born on Mount Ibuki to Yamata-no-Orochi and a human girl, another says she descended as the Dragon King Kuzuryuu's godsend. But no matter what, she is the child of a dragon god: something she shares in common with a certain Sakata Kintoki.

Just like in the past, she lives nowadays as she feels like, behaving to her heart's content.

In addition, she is also a collector of antiques and rarities, showing a keen interest in the red dragon's ulna dwelling on Kintoki's arm.

It seems that she attaches importance on rarity and the elegance of the appearance, as such she is incompatible with the King of Heroes who stresses performance and history.

(Source: Fate/Grand Order Wiki)