The Clones of Project CASTOR
- Played by: Ari Millen, Nick Abraham (body double)"Until three weeks ago, we didn't even know Project Castor existed. They were a myth. Now they're a bomb, and shock waves are coming."
A group of male clones developed in concert with the female clones of Project LEDA. While Dyad took over LEDA, the military took over CASTOR.
- Even Evil Has Loved Ones: They really do love each other like brothers and see Dr. Coady as their mother.
- Iconic Sequel Character: None of them are present in the first season. Mark, Rudy, and Miller were introduced in Season 2. Seth and Parsons were introduced in Season 3, and Ira was introduced in Season 4.
- Long-Lost Relative: They're the Leda clones' biological brothers.
- Military Brat: They were created and raised by the military. Ira is the only known exception.
- Not So Different: Lampshaded; they're often compared to Helena, another clone who's been raised to kill. However Helena herself rejects this notion.
- Promotion to Opening Titles: Their actor gets added to the series OBB starting Season 3.
- Rape Is a Special Kind of Evil: The Leda clones are horrified upon learning that Coady sent them to sterilize women through their sexually-transmitted pathogen, whether the women want to have sex or not. Mark, the most sympathetic Castor clone, averts this since the only woman he's had sex with is Gracie and she clearly consented. Furthermore, it's not even clear that he knew about the pathogen. Ira also averts this.
- STD Immunity: Inverted — every woman they sleep with is guaranteed to get the Castor pathogen passed onto them sexually. This results in pain, reddened eyes, and eventually infertility. Sarah comes into contact with the pathogen via a forced blood transfusion from Rudy, but her body manages to fight it off.
- Spear Counterpart: To Project Leda. Each of the clones are also this to a particular Leda(s), which is detailed on their respective entries.
- Tattoo as Character Type: All the Castor clones are marked by two-headed horse on in the inside of the forearm. Mark burns off his tattoo as part of his commitment to Gracie and their new life together.
- Token Good Teammate: Mark and Ira are the only particularly sympathetic Castor clones to whom we've been introduced. Parsons also comes across sympathetically, but that's mostly due to the Body Horror to which he's been subjected - we have no idea what he was like beforehand.
- Tyke-Bomb: Unlike the Leda clones, most of whom had normal childhoods and didn't know they were clones until adulthood, the Castor clones were raised together with the purpose of being the perfect soldiers. They have no other life outside the military. In season 4 we meet another Castor clone named Ira who was raised apart from the others, and thus averts this.
- Walking Spoiler: Their very existence is a major spoiler for the end of season 2.
- Your Days Are Numbered: The Castor clones have a genetic defect just like the Leda clones; only instead of targeting the uterus (since they don't any), it targets their brains. Paul and Dr. Coady have been working finding a cure since before the series started.
- "She's a miracle, I'm told."
A clone who was working for the Proletheans, although he betrays them in the end of Season 2 and runs off with Gracie. He had been sent by Project CASTOR to spy on Hank Johanssen.
- The Brute: He does the field work for the Proletheans, though he's really working for the military.
- Chekhov's Gunman: At first, it seems like he's just another goon going after the clones, but it turns out he's a clone from Project Castor. He gets a large role in season 3.
- Emergency Impersonation: He poses as Rudy to help Clone Club lure Coady away from their trail.
- Faux Affably Evil: In his first appearance, he tries to be polite to Sarah in his own creepy way, before quickly killing a Badass Bystander cook and violently trying to kidnap Sarah.
- The Fundamentalist: Subverted. He was faking his belief.
- The Gunslinger: Kills a Badass Bystander without a second thought.
- Happily Married: He elopes with Gracie in the season 2 finale. Defied in season 3, as Gracie rejects him and tries to return to the Prolethians when she discovers he's a clone, then played straight once more in the last episodes of the season.
- HeelFace Turn: Effectively betrays Hank and runs off with Gracie. And despite turning Sarah over to Project Castor, he has some serious doubts about working with Dr. Coady. This leads him to being part of Sarah's plan to turn Coady over to Dyad.
- FaceHeel Turn: In season 5, he is willing to sell out Helena for a cure to his disease so he can live his life with Gracie in peace.
- Iconic Sequel Character: Does not appear in the series until the Season 2 premiere.
- Last of His Kind: As of the end of Season 3, he is probably the last remaining CASTOR clone that we know of who is still alive. That is, until we meet Ira.
- With Ira dead, now he is the last one. Until Dr. Coady kills him, now there are no CASTOR clones left.
- The Mole: He was sent to infiltrate the Proletheans to get biological samples that Henrik stole.
- Put on a Bus: He disappears after Season 3.
- The Resenter: And for good reason too. Gracie's father inseminated her with children made from his own sperm rather than let her and Mark have children of their own.
- Shell-Shocked Veteran: He joined the military at 19 but later went AWOL. His experiences still give him nightmares. Well, he lied about going AWOL, but it's hinted that he wants to leave the military behind anyway.
- Spear Counterpart: To Sarah, Helena, and Alison. Like Sarah, he actively fights the conspiracy of his creation so it will stay away from his family. Like Helena, he is a reformed Prolethean. Like Alison, he marries someone he knows for quite some time.
- Stepford Smiler: He tries to act like everything's okay when Hank informs him that he plans to make Gracie a surrogate for Helena's children, but it doesn't last long.
- Those Two Bad Guys: Was part of one of these pairs until his partner is killed less than a minute after his introduction.
- Villainous Friendship: Initially with Gracie, but this becomes much more by the end of Season 2. By the end of Season 3, they are pretty much very deep in their relationship.
- Walking Spoiler: He is a clone, which was not revealed until the last few minutes of the season 2 finale. Even before the reveal that he was a clone, he was involved with the Proletheans.
- "Well Done, Son!" Guy: Towards Hank, who "loves him like a son." Subverted in 2.09 when Mark decides that being with Gracie and keeping her safe means more to him than pleasing Hank.
A clone who observes Helena get onto a plane in the series two finale.
- Artificial Limbs: He has a prosthetic leg, and as a result no longer takes field assignments.
- Cold-Blooded Torture: He's very casual about water-boarding Helena.
- Last Episode, New Character: He was introduced in the last few minutes of the season 2 finale.
- Neck Snap: Paul kills him with one, though not before he dished out a fatal wound himself.
- Out of Focus: Out of all the Castor clones, he gets by far the least development before he's killed off.
- The Stoic: Does not speak much, and rarely shows any sign of emotion.
A clone held captive by Marian Bowles in the series two finale. He has a large scar on his face and appears to be severely mentally unstable.
- Ax-Crazy: Rudy relishes in causing others pain and always seems to be teetering on the edge of full-on insanity.
- BrotherSister Incest/Surprise Incest: He made out with, and possibly had sex with, Krystal, presumably not knowing that she was his sister.
- Card-Carrying Villain: He gladly admits to deliberately sterilizing women by passing on the Castor pathogen through sex.
- Character Death: Crippled by Helena and allowed to die of his illness.
- The Dragon: Out of all the clones, he works closest with Dr. Coady.
- Eye Scream: Has a large scar on his face that curves around his eye.
- Honey Trap: A male example. He lures in a Leda clone named Krystal, presumably on the promise of sex, so Seth can kidnap her.
- Ill Boy: Was starting to "glitch" like Seth. He finally succumbs to it in the third season finale.
- Last Episode, New Character: He was introduced in the last few minutes of the season 2 finale.
- Scars Are Forever: Has a long, uneven knife scar on the side of his face.
- Spear Counterpart: To Helena, being the scarred and mentally-unstable clone who hunts down the Ledas. Appropriately enough, his final showdown is with her.
- Workout Fanservice: He works out naked.
First introduced in the season three premier, Seth is a clone loyal to his masters but not all that bright.
- Big Brother Worship: He's very devoted to Rudy, which Rudy uses to manipulate him.
- The Ditz: Obedient and loyal, but not smartest of the clones.
- Ill Boy: Suffers from seizure-like symptoms known as "glitching".
- Mercy Kill: During his fight with Cal, Seth's seizures get so bad that Rudy puts him out of his misery.
- Porn Stache: Has a mustache of this style.
- We Hardly Knew Ye: Dies at the end of 3.02.
- The Worf Effect: In his first appearance, he manages to get the drop on Mrs. S.
The result of Henrik and Bonnie's attempt to recreate Project Castor. Died in infancy.
A clone found strapped to a chair being kept for treatment and possibly experimentation.
- Body Horror: Parsons's entire scalp was removed and he had wires going into his brains.
- A Fate Worse Than Death: Helena finds him strapped to a chair with his brain exposed and plugged with electric wires.
- I Cannot Self-Terminate: He begs Helena to Mercy Kill him and she does so at the cost of her own escape. Then she calls out Coady for keeping him alive against his will.
- White Shirt of Death: Helena Mercy Killed him while his wearing a white hospital wear.
A clone introduced in season 4. Unlike the others, he was raised by Susan Duncan and is currently serving as a sort of guardian to Rachel and Charlotte. It's implied that the other Castor clones don't know he exists.
- The Caretaker: Susan appointed him to keep an eye on Rachel and Charlotte in an undisclosed location, which involves helping Rachel with her physical therapy.
- Character Death: He, like other Castor clones, succumbs to the clone disease in season 5.
- Comically Missing the Point: He interrupts Rachel and Ferdinand while the latter is tied to the bed, and is momentarily distracted... by the fact that Ferdinand still has his socks on.
- Deadly Bath/Driven to Suicide/Interrupted Suicide: He attempts suicide in 4x06 by hopping in the bath and downing a bottle full of prescription pills. Sarah gets to him before he dies, and he's ultimately convinced to live.
- Foil: He's essentially the "Rachel" of Castor — raised with a unique perspective of the experiment that the others are denied and thus feels detached from them. Unlike Rachel, who is an antagonist to the Leda clones until Evie Cho pushes her too far, Ira has none of the villainous qualities that most of the rest of the Castor clones do.
- Happily Adopted: He clearly holds Susan in high esteem and becomes angry when Rachel upsets her. A little too happy, it seems.
- The Last of These Is Not Like the Others: Ira is a huge exception to everything the audience learned about Castor in season 3. He was raised apart from the others by Susan Duncan, has no known involvement in the military, and feels no sense of kinship to his fellow clones.
- Long-Lost Relative: He was raised apart from the other Castors.
- Man in White: Only ever wears crisp white clothing while around Rachel. When he visits BrightBorn with Susan, he changes into a dark suit.
- Parental Incest: Susan is both his adopted mother and his lover.
- Spear Counterpart: To both Cosima and Rachael. Like Cosima, he is a scientist searching for his own cure. Like Rachel, he is raised by Susan Duncan. Additionally, while Rachel is a self-aware clone, Ira was raised apart from the other Castors.
- Undying Loyalty: To Susan. In season five he teams up with Clone Club to take Rachel down because she stabbed Susan.