The Archers of Sirius is a secret group of exorcists that is based in Europe. Their de-facto leader is Claudio Serafino.
According to what has been revealed thus far  as part of Claudio's Story Mode, the Archers of Sirius is a highly-secretive organization whose existence is not known to the public. The reason behind their sudden disclosure is not yet known at this point.
The Archers of Sirius was approached by the Mishima Zaibatsu to join their conglomerate. However, the archers refused and the Mishima Zaibatsu continues to persuade them. Claudio, the leader, prompted to investigate the reasons behind the Mishima Zaibatsu's persuasion.