Benjamin is an angel who sided with Castiel during the war against Metatron.
Background[]
Benjamin and Castiel's Guard ambushes Erza
In King of the Damned, Benjamin is at the bar with Castiel's Guard when Ezra, a Metatron loyalist, brags about knowing Metatron and his plan. Benjamin and the other angel proceed to capture Ezra for interrogation. Afterwards, Benjamin warns Ezra that he should be careful what he says in case someone hears him.
Ezra denies saying anything, but Benjamin corrects Ezra and tells him that he said "plenty". Benjamin adds that there is a consequence to Ezra's careless behavior, and much to Ezra's discomfort, Benjamin reveals Castiel in the doorway.
Castiel tries to draw information out of Ezra, but his attempts fail, leading him to call Sam and Dean. Benjamin is the one to bring the newly arrived duo to Castiel's location. Dean is visibly disturbed when Benjamin refers to Castiel as "Commander". Once the trio reach Castiel, Castiel dismisses Benjamin and informs Sam and Dean that Benjamin can be a bit "stuffy".
In Stairway to Heaven, Benjamin looks over the evidence from Oren's suicide bombing and finds a video on a phone of the event. Benjamin watches in horror as Oren commits the suicide bombing in Castiel's name. Hannah is able to identify Esther, one of Metatron's followers as the target. The two then briefly watch as the Winchesters and Castiel argue before moving it into Castiel's office.
After Dean's interrogation of Flagstaff, Flagstaff expresses her anger with Dean's behavior towards her with Hannah and Benjamin. As they talk, Dean returns with Tessa who they are stunned to discover has been injured by Dean. Dean tells the three angels that he has defused "that Old Testament graffiti" Tessa has carved into herself.
Benjamin questions Dean on what he intends to do next, causing Dean to draw an angel blade and indicate that he intends to torture Tessa. While Dean insists upon it being necessary as they need to know if there are other bombers out there, Hannah refuses. As more angels join the group, Hannah agrees to allow Dean to talk to Tessa, but states that he must leave the angel blade outside so that he won't take matters into his own hands. After Benjamin makes it clear that the angels aren't asking, Dean hands it over to Hannah.
Following Tessa's suicide, Benjamin enters the interrogation room with Hannah and finds Dean standing over her body holding the First Blade. Benjamin stares in horror at Dean's apparent murder of Tessa.
When Metatron video calls Castiel, Benjamin mans the computer and watches as Metatron states that he has survived Castiel's suicide bombing that killed Tyrus. Along with the rest of Castiel's faction, Benjamin observes the Winchesters and Castiel argue with Metatron about his actions. Metatron claims that his near-death experience has changed his perspective and offers amnesty to any angel that wishes to return to Heaven. Metatron offers Benjamin and the other angels that they can be his Heavenly Host while he will be their new God. Metatron "points out" that Castiel is willing to send angels out to die and he feels that the angels have a need to follow someone ingrained in their DNA. Metatron also reveals that Castiel's stolen grace is burning out and will soon kill him, something that Castiel had not shared with anyone. "I'm not the best, but I'm the best you've got. You want to stay with Castiel, fine. But he's playing you because at the end of the day, the only thing he cares about is himself and the Hardy Boys there. You've got a choice to make. Make the right one."
Like the rest of Castiel's followers, Benjamin is visibly convinced by Metatron's speech. After Castiel admits that Metatron is not lying about his grace, Hannah demands that Castiel kill Dean to punish him for Tessa's "murder" and to prove that he's telling the truth about everything else. When Castiel refuses, Benjamin and his Castiel's other followers abandon him for Metatron's Faction, taking the deal offered to them by Metatron.