I have been re-watching Gundam Wing recently on Crunchy Roll and after browsing around just reading up on random internet commentary on Wing and its fans, I came across several people referring to the fact that Heero basically was "Augmented" when explaining how he survives most of the injuries he does throughout the series. Re-watching the series with this in mind, a whole bunch of stuff make 100% more sense now, especially in regard to his emotions and psychology. Heero is probably part (I would take a stab at 30ish %) cyborg rather than human. Giving him extraordinary resiliency and literally superhuman strength. Probably has pain reductors within his body as well. Still human, seeing him needing oxygen, food, water, ect.

On the same subject, Trowa probably has the same conditioning if not straight up also an augmentee/cyborg as well roughly on the same level as Heero. He has superhuman reflexes that rival Heero's at times and has amazing natural agility. Acrobatics, marksmanship, and resiliency also are extremely high given his ability to survive an explosion in space at the midway point throughout the series and be alive. While he never has the same continuous medical defying events happen, he does regularly exhibit extraordinary feats of un-natural physical prowess.

Specifically for Heero and Trowa the commonalities are eerily similar: both are raised/trained by doctors, both are basically nameless soldiers (they have real names, but prefer the pseudonyms for reasons), both have superhuman abilities that exceed the other three pilots by wide margins (Quatre honorable mention... but more on that later), both are intuitively great with computers and hacking (something lacking from the other three), and both are extremely stoic and show little emotion as a cyborg nameless soldier would.

Cavate to both: both have female influence/love interests they wish to protect and actively do so in the series, both show emotions when around said lovers, both are quite open and honest with one another almost immediately. They are... literally both too much alike and therefor as close to brothers as one can have in an anime without actually stating it. That being said, both are human at the core/brain and despite conditioning still feel emotions and can suffer mental trauma (Heero suffering from a form of trauma-based Aphasia at times and Trowa's Amnesia in the series). Never mind both having weird un-natural hair that is never affected by anything!

Quatre. Throughout most of Wing, subtle hints are thrown around that Quatre is a "Newtype" or genetically engineered/artificially created. Outright confirmed in expanded lore but for just Wing and Endless Waltz you get the sense that all his large family of sisters and himself do not exactly look alike and do not bear a strong relation to his father. His genius of being able to control the Zero System, being physically stronger than most human born men, able to endure long periods of harsh environments without significant effects, has supernatural senses/perceives much better than even Heero, and natural leadership abilities all showcase the genetic tampering that has happened to him. He is by and large more of a pacifist and incomplete warrior rarely engaging in violence outside of his mobile suit. This is acknowledged by the five Gundam Engineers and to a lesser degree Heero. Despite his genetic engineering, he is not without flaws. Intelligent and Charismatic but lacking in resiliency or wisdom would characterize Quatre. The Trauma of losing his father (whether biological or not) and the support of the homeland he fought for/sacrificed for, completely broke him mentally and turned him into a psychopath through manipulation of the Zero System.

Heero and Trowa's interaction with Quatre is openly different than both of their interactions with Duo and Wufei. Both innately trust Quarte and respond to him with kindness most of the time (when he is not insane). Trowa especially opens up to Quatre (to the point that romance is uttered in the same breath) and Heero goes along with Quatre's plans most of the time without blinking an eye. When he lost his mind, both Heero and Trowa attempted to talk sense into Quatre in both of their own ways during the fight with Zero. Of the four, Quatre is the "Brains" even by Eypon Zero System /Zechs intuition and the rest of the team following his lead on most battle planning/strategic engagements (although Heero and Wufei are both tactically better). Despite basically not even being a good fighter (you literally only twice see him get into physical altercations and never use a gun outside of a mobile suit.) the rest of the team follows his commands during battles and turns to him for leadership.

Duo and Wufei (unironically two of my more favorite characters as well as Gundam's they pilot), are both decidedly human. Wufei is trained by Master O in martial arts while building a Gundam basically custom designed for his martial arts abilities and training. Wufei is never seen doing something abnormally superhuman or which cannot be explained through harsh/good training. His martial art training/hand to hand combat and tactical combat knowledge are only rivaled by Heero. Which I give the edge in both to Wufei, sans Heero's Cybernetics to bail him out of bad decisions from time to time. Duo is the same in the regard of he is 100% human and while has had some training in different skills, does not do anything superhuman throughout the series. Professor G does not have to train him as much in surviving, as he is already a top-notch survivor and scavenger. Also, among the pilots, it is known he is the best at space flight (spending most of his time piloting and scavenging). He is also as good as Heero and Trowa at stealth and infiltration, enough so that his Gundam is built with stealth, ECM's, Electronic Attack, and Jammers installed. Neither Master O or Professor G appear to be big into cybernetics or alternations like the two Doctors are in the show.

Wufei and Duo also interact with the other three differently. Wufei outright almost never talks to Quatre outside of combat. He completely does not acknowledge him in his presence. Trowa and Heero have a warrior's understanding with Wufei, but beyond that leave him alone despite spending time with him. Something which I did not touch on above but will here... it is slightly hinted a few times that Quatre is commonly mistaken as a woman. Both at the Sanc Kingdom School (when the girls through Heero was a servant and Quatre another rich noble woman) and with his gentle feminine appearance/tendencies is obviously could be mistaken for a woman by Wufei. Which Wufei openly has hostility towards most women and makes sense that Wufei would ignore Quatre.

Duo interacts with the five in a normal and at times boyish interaction but strangely you only get a bit normal back-n-forth with Wufei. Outside of Heero, he talks the most with Duo ironically enough... due to my theory of him not being another cyborg or genetically enhanced human. While Obviously Heero and Duo are a great pair and work together most of the time, it is built on a lengthy and rough start they have to work together towards a common goal. They also have a rivalry in the beginning that fed a competition between the two. Quatre and Duo get along but a subtle undercurrent of Duo constantly going along with what Quatre says both inside and outside of combat speaks to the relationship being more professional than personal. Trowa and Duo ironically have a pseudo feud/rivalry going roughly equivalent to Heero's rivalry with Duo in the beginning only it never quite got resolved after Trowa losing his memories. You still see it somewhat even towards the end. While not malicious, it is still present. This is on par with Duo having a rivalry with the two cyborgs on the Gundam Team as a natural outgoing human.

I say this about Wufei and Duo to show that they do not have a natural or easy relationship with the other three pilots but have a natural relationship with one another. Both shown by context clues, dialogue, and even hinted at in expanded material, the two normal humans have more of a bond with one another being natural loners compared with the two Cyborgs and Genetically enhanced human.

While this may or may not be cannon, I assume that Heero and Trowa are some form, part Cyborg/have cybernetics/enhancements in their bodies and the stories make a lot more sense. Quatre in expanded material has been confirmed a "Newtype" and genetically engineered.