Itsuki, Kaminari and Senrigan were a part of subdivision of the BRSD. Senrigan knew everything about Arata being affiliated to the BRSD since the time Arata started acting suspicious. But, it wasn't the same for Arata.
Senrigan didnt had the latest updates about the fight between Arata and Arsenal. And since Arata also didn't knew anything in relation to Arsenal being save by Senrigan, their friendship continued.
Meanwhile, Arsenal met a few more members of this subdivision, like Anahita. He also told them everything about The_Priest.
"Currently, he lives in the United Nations Headquarters. Nobody dares to question his real identity and no one knows how he has reached such position but... It seems like the world's government is completely being manipulated by him.