Ren Serizawa

Ren Serizawa is a major antagonist in the 2021 film Godzilla vs. Kong. He is the son of Monarch scientist Ishiro Serizawa.

He is Walter Simmons' right-hand man and Apex Cybernetics' chief technology officer, responsible for developing their psionic uplink technology for Mechagodzilla, and he's the Mecha's chosen pilot until it's hijacked and goes rogue.

Personality
In the film, little is known about Ren's personality. He's quite quiet if somewhat polite during his and Walter Simmons' meeting with Nathan Lind. He's evidently intellectually intelligent and presents a stoic demeanor like his father as well as having some knowledge of nature such as spawning salmon, but that's where Ren's similarities to Ishirō end. Ren is closely allied with Simmons' project to create their own artificial Titan and use it to usurp Godzilla as the reigning alpha Titan so that humanity will be the dominant power on Earth, and he's furthermore responsible for implementing Ghidorah's surviving skull as the core component of the psionic control mechanism for said artificial Titan due to its still-active telepathy; this despite knowing so little about the alien Titan's physiological capabilities except that its regenerative powers when it was alive were nature-defying, and despite Ghidorah's omnicidal rampage which almost saw the entire world destroyed a few years prior; implying an obscene level of arrogance, stupidity and recklessness in Ren just like in Simmons. That being said, Ren does at least have enough technical knowledge and shreds of common sense to protest when Simmons wants to immediately upload the Hollow Earth formula to the Mecha without proper testing, on the grounds they don't know how it will affect the Mecha and that Godzilla will attack them potentially quicker than they can counteract. Ren shows both hubris and a sadistic side when piloting the Mecha in a test run against a Skullcrawler, grinning in delight at his creation's power when he uses it to cleave the Skullcrawler in half, which is in stark contrast to his father's respect for nature and cautious awareness of humanity's hubris.

Ren is part of Apex's Mecha project not just because he agrees with their anti-Titan agenda, but also so he can personally kill Godzilla in revenge for his lifelong issues with his father who'd devoted his life to studying Godzilla; issues which boiled over when Ren's father's sacrifice to save Godzilla permanently dashed Ren's hopes of eventually reconciling with his father. Unlike Simmons, whose core goal is feeding his own ego, Ren doesn't care whether or not his name is remembered alongside his achievements' (perceived) contribution to future generations. It's strongly hinted that although Ren's evil actions and choices are nevertheless rooted in his past and his relationship with his father, he's been further corrupted by repeated exposure to Ghidorah's consciousness remnants through the psionic uplink when testing Mechagodzilla's parts.

Ren is a humanist who believes he is siding with the human race by conspiring with Apex. He observes that humans have triumphed over (non-Titan) predators and disease via increasing invention, ingenuity and domestication since mankind's beginning, and he believes the Titans are not gods but are nothing more than another kind of animal to be conquered the same way. Ren thinks leaving the fate of humanity in Godzilla's hands by letting him fight off the hostile Titans is unacceptable and is a sign of callousness in those who support the Alpha Titan, and he believes Godzilla deserves to die for the thousands of casualties that have occurred in Titan battles, yet he's a hypocrite who's blind to the fact that he and Simmons knowingly putting millions of people at unnecessary risk by causing Godzilla's rampages on Apex facilities in population centers contradicts this.

Somewhat similar to his father's awe and admiration for the Titans, Ren is a nightmare fetishist: he greatly admires Skullcrawlers' purity as vicious killing machines built solely to eat, and he apparently had a "love at first sight" reaction when Simmons first showed him Ghidorah's skull, and it's further confirmed that Ren never once thought that harnessing Ghidorah's consciousness remnants might backfire on him and Simmons catastrophically. It can be argued that Ren's nightmare fetishism combined with his sheer pride and hubris are what cause his and Simmons' doom after he meddles with and underestimates the remaining power in Ghidorah's remains.

Godzilla
Ren is allied with Apex's plan to kill Godzilla specifically using Mechagodzilla, in order to replace him with a human-controlled artificial Titan as the reigning alpha; and as the Mecha's pilot, Ren is the one expected to fight and kill Godzilla via the Mecha. In the film, very little is shown of Ren's feelings towards or opinions on Godzilla beyond this; but when Ren and Simmons are in the process of evacuating the Pensacola facility while Godzilla rampages towards them, Ren takes a moment to watch the Titan from afar with an expression that looks strangely like a mix of anger and pain.

Ren's feelings towards Godzilla are fully explained in the novelization. Before his father's death, Ren used to compare Godzilla to an older brother who Ren's father completely neglected Ren to dote upon. After his father's death, Ren uses Godzilla as a focus for all his rage and resentments concerning his father, and he intends to kill the very same creature his father saved in revenge for all of that. Ren also (hypocritically) perceives Godzilla as a monster who deserves to die for causing thousands of casualties whilst saving the world from hostile Titans in the past.

Ishiro Serizawa
Ren's father was frequently absent from his life due to committing so much of his time to his work for Monarch studying the Titans. At first, Ren tried to get his father to notice him and show him approval, to no avail. When Ren's mother died whilst Serizawa was away on work, and Serizawa simply told Ren that she understood his work (something which Ren did not, and he blamed his father for), the rift between father and son expanded. Despite this, Ren always hoped that his father would experience satori and reconcile with him until his father's death prevented that from ever happening.

Ren considers his father the only person he ever cared to impress. By the time of Godzilla's Apex-instigated rampage, Ren is contemptuous of his father's naturalist view and high opinion of Godzilla and his actions, thinking his father's treatment of him coupled with his stance on letting the Titans fight are signs that Ishirō never cared about his family or human beings generally. Ren is aware that his actions go against every single value that his father ever stood for, and he's doing it somewhat deliberately as an expression of rage and rebellion against the father who Ren believes put his work before his family and before human lives and whose ideals Ren believes are what got him killed.

Ren's mother
It's mentioned in the novelization that Ren kept the depths of his resentment towards his father secret even from his mother when she was alive. Her death caused the rift between Ren and his father to expand significantly when Ren was forced to organize her funeral himself as a teenager and Ishirō didn't make time to return home until two days after the ceremony.

Walter Simmons
Ren is Simmons' right hand, and the two work closely together on the Mechagodzilla project. They're frequently seen together or at least in the same Apex building, and they approach Dr. Nathan Lind together with their proposal that he help them reach the Hollow Earth. Ren is something of a voice of reason to Simmons' overconfidence, openly contemplating the possibility that Maia, Lind and Andrews will fail in their mission to deliver the Hollow Earth's energy formula to them, and also protesting when Simmons impulsively wants the energy formula uploaded to Mechagodzilla immediately without proper testing due to them having no idea how it could affect the Mecha or the Ghidorah skull. However, still Ren submits when Simmons impatiently orders Ren to "get in the goddamn chair".

The novelization reveals that despite Walter's respect and fondness for him, Ren actually is merely using Simmons to further his own agenda and sees Simmons as a mere stepping stone. Ren secretly views the petty egotist as a "chattering baboon" who is good at taking credit for others' invention, and Ren is seriously considering disposing of Simmons the moment he no longer needs him.

History
Ren's background is revealed in the novelization. His father Ishiro was frequently absent from Ren's life when he was growing up, repeating the same parenting pattern that Ishirō's own father Eiji took with him: sacrificing most of his bonding time with his son to commit to his work for Monarch tracking and studying Godzilla. Ren worked hard, developing his skills with technology, in an attempt to show his father what he can do and make his father notice him. A major turning point which saw Ren's issues with his father dramatically worsen was when Ren's mother died while his father was away on an expedition: Ren, only eighteen years old at the time, was forced to organize his mother's funeral himself, and his father didn't return until two days after the ceremony, simply telling Ren that his mother had understood what he was doing. Ren however, never understood his father's work or his reverence for the Titans. Despite his fractured relationship with his father and contempt for the man's pro-Titan attitudes, Ren still hoped his father would one day realize what he'd been neglecting and it would lead to reconciliation between them. However, in 2019, his father's heroic sacrifice to revive Godzilla so the latter could save the world from King Ghidorah's Titan rampage permanently dashed Ren's hopes.

Ren subsequently became part of Apex Cybernetics' secret project to build an artificial Titan of their own with which they could kill Godzilla and usurp his dominance over the other Titans to make humanity the new alpha species, developing the Mecha technology, and acting as the pilot of the Mecha's psionic uplink who tested out Mechagodzilla's parts with the uplink piece by piece.

Godzilla vs. Kong
When Godzilla launches his first attack on the Apex facility in Pensacola, Florida after sensing Ghidorah's corrupted signal emanating from there due to Mechagodzilla's parts, Ren and Walter Simmons are seen quickly evacuating by helicopter. Ren however takes a moment to glare at the approaching Titan from afar before Simmons tells him they need to go and he joins him.

Afterwards, with Godzilla's attacks on Apex facilities in the middle of population centers having the whole world convinced that Godzilla has turned against humanity, and Simmons taking advantage of this misconception to make it look like Apex will save humanity by unleashing Mechagodzilla instead of being the ones entirely responsible for the calamity in the first place; Ren and Simmons together approach Nathan Lind for part of the next stage of their plan. They ask for Lind's help in accessing the Hollow Earth (based on Lind's research on the Hollow Earth and his and his late brother's previous failed attempt to enter it) and harvesting an energy source Apex's satellites have detected there which is almost identical to Godzilla's own energy - they claim Apex will use the energy source to fuel a weapon capable of stopping Godzilla, and unbeknownst to Lind, they're seeking the energy source because Mechagodzilla drains any other power source too quickly to operate for longer than a few minutes and they believe the Hollow Earth energy will fuel the Mecha lastingly. Ren and Simmons also present the Hollow Earth Aerial Vehicle so that a successful entry is possible.

At Apex headquarters in Hong Kong, Ren pilots Mechagodzilla in a test run against an adult Skullcrawler. He unknowingly saves Madison Russell, who has infiltrated the fighting arena, from being eaten by the Skullcrawler when he uses the Mecha to grab and easily restrain it. Ren kills the Skullcrawler by cleaving it in two with the Mecha's Proton Scream, before the Mecha runs out of power.

Later, when Maia Simmons successfully transmits the Hollow Earth energy formula to the Hong Kong HQ, with Godzilla entering the city outside the HQ after having detected the Mecha's signal from the previous test run; Simmons eagerly tells Ren to get ready to start up Mechagodzilla. Ren protests, pointing out that Godzilla is already much too close to their HQ for comfort and will come straight for it the moment Mechagodzilla comes online, and expressing concern that they shouldn't upload the energy source to Mechagodzilla without proper testing due to the possibility of unforeseen side-effects, but an annoyed Simmons orders him to "get in the goddamn chair" anyway, with Ren reluctantly following through.

As Ren returns to the Skull Room and activates the psionic uplink to Mechagodzilla, Ghidorah's subconsciousness overrides the systems and hijacks the empowered Mecha, severing Ren's uplink as Ren looks around the Skull Room in concern. Moments later, whilst Mechagodzilla becomes sentient and kills Simmons, the Ghidorah skull draws in a massive electrical surge which hits Ren, and he collapses apparently dead.