Between Inhumans and portals to alien planets, the "Agents of S.H.I.E.L.D." have handled their fair share of the unexplained, but they've never encountered something quite like this. When Ghost Rider roars onto the show, the series -- as well as the Marvel Cinematic Universe at large -- will enter uncharted territory; after all, everything they've encountered so far can be explained away by science, but Ghost Rider falls on the mystical side of the Marvel U. As out of left field as his "Agents of S.H.I.E.L.D." debut may feel, however, the character has one surprising tie to the show: Daisy's father Dr. Calvin "Zabo" Johnson, aka Mr. Hyde.

Let's back up a bit. Robbie Reyes, the Ghost Rider revving up for his appearance on "S.H.I.E.L.D.", is actually something of a newcomer. Created by Felipe Smith and Tradd Moore, "All-New Ghost Rider" dropped its first issue as recently as 2014. Robbie dies in this very same issue, only to be brought back to life by the spirit of his uncle-turned-Satanist Eli Morrow. While Zabo doesn't have a direct hand in Robbie's death, he is the reason behind the murder. When Robbie encountered Zabo's men, they were after Zabo's latest creation -- a batch of pills that trigger temporary, superhuman enhancements in the average human -- in the back of the Dodge Charger that Robbie had "borrowed" for a drag race. Zabo's men then killed Robbie in order to eliminate any possible witnesses, leading to Robbie's resurrection by Morrow, whose spirit inhabited the car. Even after Robbie's rise as the newest Ghost Rider, Zabo continued to pose a remorseless threat to Robbie's world, acting as the primary antagonist for ten out of the series' twelve-issue run.

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Of course, "Agents of S.H.I.E.L.D." won't adapt Robbie's origin by the book. In a recent interview, executive producer Jed Whedon revealed, "We can't say much about what we're going to do with [Robbie Reyes'] origin, but we are doing our own spin on it. We don't want people who have read the comics to know exactly what's coming." As such, it's pretty reasonable to rule out Zabo as the primary antagonist of Season 4 -- but that doesn't mean we should rule him out altogether. In the same interview, Whedon also expressed interest in bringing Kyle MacLachlan's character back in Season 4; told viewers would like to see MacLachlan's return, Whedon replied, "So would we."

The show's version of Zabo is also a lot more similar to Ghost Rider than his comic book counterpart. Like Zabo, Robbie struggles with a rage that triggers a transformation into an almost uncontrollable otherworldly being; Morrow also tends to bring out the worst in Robbie, in much the same way Mr. Hyde sets Zabo on a course of utter destruction. In the end of the day, though, both of them care about one thing above all others: family. Where Zabo tore towns apart in his search for his daughter Daisy, the comic book version of Robbie Reyes has managed to get Morrow under control through the power of his love for Gabe, his developmentally challenged little brother. Of course, the two took drastically different paths when it came to their expression of this love, but they bring the same kind of passion and loyalty to their family members. If the show brings Zabo back alongside Robbie, the two would serve as a nice parallel to one another, and the memory of Zabo might even humanize Ghost Rider for Daisy, Coulson and crew -- or not.

"We hit on Ghost Rider and... and in particular with the character of Robbie Reyes, because our show really at the end of the day hinges on the idea of family and how that works in the world," Head of Marvel Television Jeph Loeb explained at Comic-Con International in San Diego. "Robbie and his relationship with Gabe and his relationship with the Ghost Rider are all really interesting things for our S.H.I.E.L.D. folks to encounter, possibly even in negative ways."

Nevertheless, the similarities between Ghost Rider and the show's version of Zabo are clear, and Zabo is an important piece of Ghost Rider's origin. Even though the show will go a different route with this origin, it could certainly incorporate Zabo in a different way. For instance, in "All-New Ghost Rider," Johnny Blaze stopped by to mentor Robbie in the ways of Ghost Rider (after an obligatory first fight, of course). Zabo would easily fill a similar -- if not identical -- function, as Johnny Blaze does not yet exist in the MCU and both characters suffer from rage they struggle to control.

However, this would require Zabo to remember his history as Mr. Hyde, since he was (voluntarily) brainwashed into forgetting that period of his life in the Season 2 finale -- but this could be easily rectified. "Agents of S.H.I.E.L.D." has played with this idea of memory since its first season, and the organization even has a device that dredges up the past whether the subject likes it or not. Alternatively, Zabo could take a page out of Robert Louis Stevenson's book by blacking out uncontrollably and experiencing episodes as Mr. Hyde. That would certainly be enough cause for a reunion with Coulson and his team and perhaps even set him on a crash course with the Ghost Rider. After all, S.H.I.E.L.D. must be monitoring his progress.

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The core of Zabo's character and alter ego also boils down to the idea of superhuman enhancement, which is something the show will continue to explore in Season 4 with one Mr. Holden Radcliffe. "We've dealt with normal people who want to... be more than just a man, like Mike Peterson. With Coulson, he has a robot hand," executive producer Maurissa Tancharoen explained. "Then we meet [Holden] Radcliffe, who's definitely into body modifications. This is just sort of a progression and extension of that."

While Holden has been exploring the mechanical side of superhuman enhancement with the creation of Life Model Decoys (LMDs), he hasn't shied away from dabbling with genetics in the past. In Season 3, he inadvertently created Alpha Primitives when Hive demanded that he recreate the process the Kree used to transform regular humans into Inhumans -- but that isn't to say he was an unwilling participant. Though his crimes were ultimately dismissed, he was all-in for Hive's plan to turn average humans into Inhumans. Now that he's working with S.H.I.E.L.D., he could have access to Zabo's formula, something he'd be eager to get his hands on. If Radcliffe does attempt to recreate Zabo's Hyde formula, it could lead to a similar Ghost Rider origin with a distinct "Agents of S.H.I.E.L.D." spin. What's more, it could bring Zabo back into the fold as the only true expert in this field. If not Radcliffe (who must be very busy with his LMDs), a new villain could very well rise in his place, with the same purpose and consequences.

Regardless of whether or not "Agents of S.H.I.E.L.D." chooses to explore Robbie Reyes' connection to Zabo and Mr. Hyde, fans are clamoring for MacLachlan to make a return -- and this is the perfect opportunity to do so. There are many ways that Zabo ties into the upcoming season's themes of superhuman enhancement and family, and the character would make an excellent parallel for Ghost Rider himself. What's more, it would give Daisy's Season 4 arc a very personal spin, as she would find herself between her father and a strange new being. Here's to hoping that Zabo feels a little more like himself when "Agents of S.H.I.E.L.D." returns!

Starring Clark Gregg, Chloe Bennet and Ming-Na Wen, "Agents of S.H.I.E.L.D" returns Tuesday, September 20 at 10 pm EST on ABC.