Bobby Singer and Castiel have a working relationship as well as a mutual understanding. They both want to protect Sam and Dean. Although they do not work often together, Bobby and Castiel do protect each other. Like Sam and Dean, Bobby calls him "Cass". This friendly relationship extends to Castiel and the Apocalypse World Bobby.
Bobby first heard of Castiel, during the séance of Pamela Barnes to discover what freed Dean from Hell. Bobby seemed to realize that Castiel was no ordinary supernatural creature after Pamela's eyes were burned out.
In Lazarus Rising, they first meet when Bobby and Dean summon Castiel to try and figure out who he is. When Castiel arrives, Bobby fires everything he has at Castiel, only to have Castiel keep walking towards him. Bobby is surprised to see Castiel unaffected, especially when Ruby's Knife doesn't work. When Castiel gets near Dean, Bobby quickly attacks him only to be countered and knocked out by Castiel with a mere touch. Castiel assures Dean there is no harm done to his friend.
In this season, Castiel and Bobby have more interaction and come to see each other as friends since they both seek to protect Sam and Dean.
|“||Are you really gonna bitch -- to me? Quit pining for the varsity years and load the damn truck.||”|
in Two Minutes to Midnight
When Castiel appears after he has lost his legs, Bobby demands that Castiel heal him but Castiel can't due to being cut off from Heaven. The first major project Castiel and Bobby work on together is when everyone gets ready to hunt down Lucifer after receiving the Colt. Castiel stays at Bobby's place everyone gets ready there. Before leaving for the mission they take a group photo. During the mission Castiel wonders off, and this gives Bobby the hint of what's going on. After the hunt has failed, Castiel and the others return to Bobby's.Two Minutes to Midnight. Also a comic relief moment between the two when Bobby tells them about a storm about to wipe a city from the map. By now, Bobby is in a wheelchair and Castiel has lost all his power. As they load the van, Castiel begins to feel sorry for himself and is feeling useless. Bobby gets angry and tells Castiel that it ain't bad, and not to preach to him about feeling useless. Bobby is still hurting from being in a wheelchair. Finishing off his speech to Cass, Bobby tells Castiel to shut up and load the truck.
Very soon, Crowley restores Bobby's use of his legs and the latter is able to resume active hunting. So they set out to stop the shipment of Croatoan Virus. Everyone does their part. Castiel finally figures out how to work a gun, and uses it to save Sam's life, which surprises and pleases Bobby. Once they have everyone out, they blow up the building.
Then in Swan Song, they work together to help Sam gather demon blood so he can house Lucifer. As Sam leaves, Bobby and Castiel say their goodbyes, and watch him go. Both wish Sam to succeed, but they know there is a small chance of success. After Sam fails, both Castiel and Bobby lose hope. They both try to stop Dean when he wants to go after Sam, but Dean doesn't listen. As Dean leaves, Castiel and Bobby realize they need to help, as they exchange a look. They arrive at Stull Cemetery where Lucifer and Michael are, and Castiel throws a cocktail of holy oil at Michael. Lucifer kills Castiel as a result, Bobby soon follows after desperately attacking the archangel.
Soon after the battle and the imprisonment of both Archangels, God soon resurrects Castiel with all of his powers restored and additionally upgrades him to the rank of Seraphim. Castiel's second act upon revival (his first being to heal Dean) is to resurrect Bobby who is shocked by his appearance, though grateful to be alive again. This is their last meeting for a year later.
|“||It was a bad idea letting them go." "Come on. You don't let Sam and Dean Winchester do squat, they do what they gotta, You know that. Anyway, we want Eve, we need coordinates. So we can sit here bellyaching or, we can poke that pig til it squeals.||”|
— Castiel and Bobby
in Mommy Dearest
Due to Castiel fighting a civil war in Heaven, Bobby doesn't see him as much as Sam and Dean do. Bobby does hear stories about Castiel from the boys though. When they find a way to kill Eve, they call Castiel.
In Frontierland, Castiel sends the boys back in time. As Castiel leaves, he tells Bobby to pray for him in 24 hours so he can get Sam and Dean back. If it's longer than that, Castiel would no longer be able to find them. Castiel later returns to Bobby's house injured and needing a safe place, and draws a sigil on Bobby's wall in his own blood. As soon as Castiel stands up, he passes out and Bobby manages to catch him. With an hour remaining, Castiel wakes up and Bobby asks what happened. Castiel tells Bobby that Rachel betrayed him and continues on by saying that he will heal soon. Although Castiel tells Bobby that because of his weakened condition, he doesn't have enough power to bring Sam and Dean back. Bobby is shocked at the news, not to mention angry. Castiel tells Bobby that he could power up if he touches Bobby's soul. Castiel further explains that if he doesn't do it right, Bobby could explode. Knowing the risks, Bobby decides to trust Castiel and agrees to do it. Castiel draws power from Bobby's soul and brings the boys back. This process does cause Bobby pain.
In Mommy Dearest, the hunt for Eve begins. When they have no leads on Eve, Bobby tells Dean to summon Castiel, since Castiel listens to Dean. When Castiel arrives. he also reveals he can't find Eve. Once they do have the location they head out. Once there, Castiel discovers he is rendered powerless. When Sam and Dean leave to bring some orphans away, something Castiel isn't too happy about. Bobby isn't impressed when Castiel begins to complain about it. Bobby tells Castiel that no one can control the Winchesters, and they still need to find Eve. So they begin to torture the captured Jefferson Starship who's posing as the Sheriff in the town. When Bobby gets no results, Castiel steps in, and gets the answers. This triggers Bobby's suspicions on what Castiel is really doing. Bobby and Castiel are both captured by Eve and helpless to watch her talk with Sam and Dean. After Dean killed Eve, Bobby watches as Castiel regains power and kills her forces, voicing he needs to go on more hunts with them. At the end, Bobby admits his concern about Castiel and believes that he is lying to them since Crowley was alive.
So they begin to investigate where Crowley is, but don't tell Castiel in The Man Who Would Be King. Bobby makes sure no one is telling Castiel what they are doing. Unknown to Bobby, Castiel is spying on them, and kills any leads before they can trace it back to Crowley. Castiel even kills the demon counterpart of Bobby, Ellsworth. When they arrive they find the place clean, and Dean tries to convince Bobby that Castiel hasn't betrayed them, but Bobby sees no other way, and says:
|“||If there's a snowball of a snowball's chance here, that means we're dealing with a Superman whose gone dark side. Which means we've got to be cautious, we've got to be smart, and maybe stock up on some Kryptonite.||”|
— Bobby about Castiel
in The Man Who Would Be King
In Let It Bleed, both Bobby and Castiel race against each other to find how Purgatory was first opened. Bobby seemed to lag behind Castiel a bit, until he saw the picture of the maid at H.P. Lovecraft's party the night the portal opened. He figures out that it was Dr. Visyak, his old flame, and goes to protect her. Although she refuses his protection, and is eventually captured by Castiel.
Following that, In The Man Who Knew Too Much, any friendship or trust Bobby had for Castiel is gone when he finds Dr. Visyak in an alley, and he finds out that Castiel tortured her. Bobby tries to attack Castiel, but Dean stops him due to Cass being more powerful than Bobby. When Cass knocks down Sam's wal, its Bobby who wants to go after Castiel to stop him. Unfortunately, they are too late, and Castiel proclaims himself God. Making Castiel, himself an enemy to Bobby.
|“||Well, he was friends with us, wasn't he? You can't get much dumber than that.||”|
— Bobby about Castiel
in Hello, Cruel World
Bobby was first to bow to Castiel, but due to the threat Castiel impossed on their lives. Bobby approaches Dean on how to deal with Castiel, but Dean has no plan. So Bobby doesn't pursue trying to find a way to stop Castiel. Then the idea to bind Death comes along, Bobby and the others head out. But that plan fails, when Castiel unbinds Death. Although Death tells them they need to get Castiel to let the souls go. Which Bobby doesn't think will happen.
Although Castiel later shows up on Sam's prayer asking for help. So Bobby helps Castiel and gets the portal ready. When its about to start, Bobby helps Castiel over to the portal. After Castiel lets the souls go, the Leviathans stayed in Castiel. With the vessel failing in Hello, Cruel World, they head to the water supply and Castiel disappears in the water. Bobby admits to being friends with Castiel, when they find his trenchcoat.
In There Will Be Blood, even though Castiel and Bobby haven't seen each other, Bobby knows Castiel is alive and suggests to ask Castiel for help.
In Survival of the Fittest, while they don't interact, Castiel watches as Bobby is put to rest and is shown to be saddened by it.
In Inside Man, when needing to break into Heaven itself, Castiel agrees to call upon Bobby for help. Bobby, though feeling himself to be rusty, agrees and after he gets Castiel into Heaven, the two work together to free Metatron. Though they don't act anything more than formally towards each other, Castiel tells Bobby about Dean at his request and questions if he'd given up trying to find a cure for the Mark of Cain like Sam refuses to. Bobby gets in deep trouble for helping Castiel but finds it worth it. In return, Castiel does the favor of delivering a note to Sam from Bobby.
In All Along the Watchtower, when Castiel travels to an alternate reality, his life is saved from a Tempter Demon by an alternate version of Bobby. Castiel is shocked to see this version of Bobby who in turn recognizes Castiel as different from the angels of that reality and spares his life rather than killing Castiel as Bobby would usually do with an angel. Bobby fills Castiel in on the events of the alternate reality and Castiel later tells the Winchesters he learned about it from "a friend" when explaining his knowledge to them. Castiel eventually brought the Winchesters to meet the alternate Bobby who acted amicably towards the group, including Castiel, despite not understanding why they knew him.
In Jack in the Box, Bobby and Castiel share a bit of friendly banter at Mary Winchester's hunter's memorial over how Mary would've enjoyed the presence of a Wraith at the memorial whom Bobby had killed.
In The End, it is shown that Castiel and Bobby both were in the resistance against Lucifer and defending people from the virus victims. Unfortunately, Bobby was killed in action, and a powerless Castiel turned into a hippie.