The Flash’s Killer Frost and Marvel’s Hulk share creative DNA as retellings of the classic Jekyll and Hyde dynamic, although one thus far has a much happier conclusion. The two characters share similar journeys dealing with the complications of two being cohabiting one physical form, but their stories lead to entirely different outcomes, with Killer Frost’s showing a more thoughtful, complex story.

In the Marvel Cinematic Universe, Hulk’s storyline has focused on how Bruce Banner handles his alter ego. After activating his Hulk side, Banner hid off the grid until he learned how to control him. At the end of Avengers: Age of Ultron, concerned he might hurt someone, Hulk leaves Earth. He and Hulk return to their on/off body sharing after Banner is awakened during a reunion with Thor during Thor: Ragnarok. At the beginning of Avengers: Infinity War, Thanos overpowers Hulk, which traumatizes Hulk to the extent he doesn’t reemerge from Banner for the rest of the film.  Avengers: Endgame reveals that Banner has become Professor Hulk, a calmer iteration of the green giant that is primarily Banner’s brains with a slightly smaller version of Hulk’s brawn, showing that instead of trying to deal with his alter ego, Bruce resulted to using science to gain the physical advantages of his counterpart.

Related: Why Flash Stopped Giving Caitlin Love Interests (& Focused on Killer Frost)

Caitlin Snow’s relationship with Killer Frost mirrors Banner and Hulk’s partnership, though there are some key differences. Following one of Barry Allen’s attempts to repair timeline damage, Dr. Caitlin Snow discovered she was a meta-human, Killer Frost, and shared abilities with the icy Earth-2 doppelganger whom she feared. Like Banner, the emergence of her other self was triggered by anger. The scientist in Caitlyn sought ways to cure herself, something Banner also pursued early in Hulk’s existence. Caitlin ultimately declined a possible cure, believing she had control over Killer Frost. Later, when Frost fought back, Caitlin chose to attempt an alliance, rather than continuing their constant battle for control.  After Caitlin thought she had lost Killer Frost due to an attack, the pair agreed to share their body with each other more equally - learning to coexist, instead of continually warring for control - and providing an arc that is far more satisfying than the MCU equivalent.

Marvel barely explores how Banner and Hulk relate to each other. Hulk is often viewed by those around Banner as inferior; Hulk’s Avenger teammates often handle him with caution, and Tony Stark even created the Hulkbuster suit solely for the purpose of restraining Hulk. Hulk’s trauma from Avengers: Infinity War is likewise never dealt with onscreen, while, on The Flash, Killer Frost is shown processing the effects of the attack she survived. Professor Hulk seems to almost erase the separate personality of Hulk, leaving Banner as the primary benefactor of his alter ego’s abilities. Given that the most interesting aspect of the Hulk is the warring personalities of Banner and his green counterpart learning to deal with one another, this feels like something of a hugely missed opportunity so far.

Flash Barry Killer Frost

Caitlin and Frost are two distinct individuals with distinct personalities that share a body, and they have actual conflict and resolution with each other, which translates into a bond that feels earned. Their ongoing collaboration and banter draw its appeal and strength from the mutual respect they have for their other half’s distinct needs. Frost at one time laments about not having a social life, and Caitlin gives her alter ego extra time in the driving seat to bond with Iris West and her team.

The Flash's Killer Frost's story is distinguished from Hulk’s by how it gives equal weight and power to the two individuals involved. Caitlin and Frost worked together to determine a way to coexist, but Banner and Hulk's decision that they couldn't exist separately was presented too abruptly to pay off their individual arcs up to that point.

Next: How Black Lightning Could Become The Flash's New Arrowverse Mentor