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Standing in contrast to his father, Ren was Ishirō's complete opposite: a hubristic mad scientist who had a personal hatred for Godzilla and was obsessed with killing him, and who aided Apex's efforts to make humanity (with themselves at the helm) the sole dominant species over all the Titans with nil regard for the millions of civilians they knowingly put in danger, and who grinned in sadistic relish when using Mechagodzilla to rip Skullcrawler Number 10 in half. Compared to his boss Walter Simmons, Ren is much more quiet. However, despite his nightmarish hubris and lack of scruples, and his extreme reckless stupidity in using Ghidorah's partly-alive remains in Mechagodzilla, Ren was one to err on the side of caution when it came to the mysterious Hollow Earth energy source; as even he was perturbed by the vast potential for unexpected side-effects it would have on the Mecha, and by Simmons' reckless willingness to activate the Mecha when Godzilla was right outside their base and could kill them all once he sensed the Mecha before they could get the Mecha outside.
The novelization expands more on Ren's motivations. Truthfully, Ren was deeply resentful of his father for sacrificing his relationship with his family to pursue his Monarch work and devote himself to studying Godzilla, and consequently felt a sharp disdain towards the Titans for taking up all his time and attentions - Godzilla in particular Ren views as a disposable elder brother whom took his father away from him. In further stark contrast to his naturalist, human-criticizing and Titan-revering father; Ren views the human race as the supreme force on Earth whom are always growing in power, knowledge and mastery of their world, and he believes the Titans are nothing more than another natural obstacle like prehistoric humans' predators and diseases that are meant to be conquered the same way. Though Ren views himself as a humanist who is helping humanity to fulfil their destiny, he's a hypocrite; who justifies his efforts to murder Godzilla by saying the latter has caused thousands of casualties, when Ren himself never once thinks of the millions of people Apex are deliberately and needlessly putting in Godzilla's warpath, and he seems oblivious that his Mechagodzilla project if successful will still cause casualties while fighting off Titans just like Godzilla inevitably does. Ren is also shown to be rather self-serving and conniving with a god complex, as he only chose to work under Simmons at Apex for the sake of ascending his mortal coil to become a physical god through Mechagodzilla, and intended to murder his insufferable supervisor using their shared creation once Simmons was no longer needed.
Ren views Godzilla as much of a threat to humanity as any other Titan as his colleagues and supervisor, Walter. But his enmity towards the ancient being ran much deeper than a prideful sense of predominant egotism as with his financial backer or the fear-induced cruelty and vehement rebuttal of the godly giant like other humans. The younger Serizawa longed for the attentions of his parental figure as a child but always found himself playing second fiddle to his greater passion for Godzilla; the work study of which often taking precedence over his home and family life. This instilled within Ren a deep seated hatred of the King of the Monsters, with Ren putting all of his neuromechanical expertise to finishing tests and practices upon Apex's faux Titan for the express purpose of deposing and killing the item of his ire in a bid to one-up what he feels took his father away from him. When viewing Godzilla's advance on the Apex Pensacola facility, Ren takes a significant moment to stand and watch the Titan from afar with a glare of anger and pain; a dark mirror-image of how Ishirō would often look upon Godzilla with reverent awe.
Walter relies on Ren as Apex's chief technology officer to make Mechagodzilla work using Ghidorah's remains, and the two work closely together and are often seen together. In many ways the two seem to have a strange working relationship with one another, Walter viewing himself as an eccentric father figure to the more cautious and uneasy Ren. Despite Simmons' trust in Serizawa, Ren quietly displays a sense of weariness towards his employer's plans. All of that being said, when Ren protests to an overenthusiastic Simmons rushing the use of the Hollow Earth energy formula upon Mechagodzilla's completion without further testing, Simmons reacts like an impatient child annoyed at an elder for interrupting them, quickly losing his patience and telling Ren to "get in the goddamn chair [referring to the Mechagodzilla-piloting cockpit]".
In the movie, Ren Serizawa pilots Mechagodzilla for the sinister corporation Apex Cybernetics, and uses a psionic uplink fashioned from the skull of Ghidorah's leftover severed left head to do so. Ren kills a Skullcrawler codenamed "Number 10" for target practice while controlling Mechagodzilla. When Apex obtain the Hollow Earth energy source's formula and can synthesize it to fully empower their Mecha, Ren protests to Simmons' impulsive decision to immediately upload the new power source to Mechagodzilla without even basic testing, and while Godzilla is so close to their headquarters that they'll have a dangerously narrow time window to activate the Mecha and get it outside of the HQ before Godzilla senses it and attacks. When Ren attempts to link his mind with the empowered Mechagodzilla, the system malfunctions as the new power source prompts Ghidorah's subconsciousness in the skull to hijack the Mecha for itself, severing Ren's link to the machine before a violent power surge in the skull electrocutes him.
Ren's death in the official novelization is different from in the movie. Whereas the movie explicitly shows Ren's connection to Mechagodzilla was severed and he bounced back into his body when Ghidorah's subconsciousness infected the Mecha; in the novel, Ren's mind gets stuck inside Mechagodzilla but unable to control it, before Ren is promptly overwritten by the Ghidorah-derived consciousness. It's also revealed in the novel that Ren experienced a "feedback loop" between his body and the Ghidorah skull when Ghidorah's consciousness began taking control, and the novel later simply says when Madison returns to the destroyed Skull Room, "She found no sign of the pilot."