Katsuki loves to succeed above anything else, he always has. He revels in the thrill of competing, of winning and he basks in the resulting praise he receives about how incredibly he performed.
His entire life everyone from family to peers to strangers had praised Katsuki for his physical prowess, powerful Explosion Quirk and academic excellence.
It never once occurred to him that he would become anything other than the Number One Hero - on par with his idol All-Might. It still hasn't. Not even after shitty Deku's miraculous "discovery" of a powerful latent Quirk and his subsequent admission to UA. Not even following the first-year Quirk Apprehension Test when he ranked third. Not even after hearing his own mother calling him weak and blaming him for getting kidnapped in the first place. Hell, not even after failing the Provisional Hero Licensing Exam.
These failures hurt more than anything, but Katsuki remained focused on achieving his goals and becoming a successful hero. The Number One hero. He wants to win like All Might and no matter what anybody says that will never change. Being a hero was where Katsuki was going to come out on top. He has to. After all, he had already learnt the painful lesson that in certain situations he could never be anything besides below average.
Katsuki is self-aware enough to recognise he has an inferiority complex and he massively overcompensates because of it. Sometimes when he is alone and has nothing else to distract him, he wonders to what extent his desire for perfection and his abrasive, short-tempered personality is shaped by the inadequacy he feels every time he stands naked in front of the mirror.
The thing is, Katsuki's penis is more than just small. What he has is a genuine, clinically diagnosed micro-penis. It disappears into Katsuki's pubic bone when soft and barely reaches four centimetres when erect.
It had been this way for as long as he can remember. Katsuki is not sure when he became aware that he was different. Perhaps it was in the locker room showers after gym class when he noticed. Or perhaps it was when a well-meaning paediatrician, pulled aside his parents and recommended Katsuki commenced a course of steroid injections to induce penile growth.
Katsuki rarely remembers what he considers irrelevant information about the extras he interacts with but for some reason he can visualise that paediatrician, with her overly plucked brows and a Quirk that gave her the characteristics of a deer, in stunning clarity.
Throughout his life, Katsuki continues to oscillate between his feelings of superiority and inferiority until they become impossibly intertwined. These emotions form an intricate psychological complex that Katsuki's behaviour exhibits in increasingly destructive ways.
As a child, he bullied Midoriya Izuku. Most people assumed it was because he was Quirkless. Nobody imagined the true reason was Katsuki's frustrated inadequacy stemming from a wholly different matter. Back then, Katsuki ignored his own weakness and ended up bullying Midoriya because he was an easy target. Although Katsuki's young mind was incapable of parsing through his own emotions and this understanding did not come until later, a part of him always hated his inclination to be cruel. As somebody who worshipped All Might, harming anybody other than a villain was the most unheroic thing he could think of. But Katsuki had so much anger and nowhere to direct it. It just felt so fucking good to punch something. To hurt something.
In middle school he was still full of resentment but the added influx of hormones created a dangerous combination. Puberty was a confusing time for Katsuki and he alternated between simmering anger and overwhelming arousal. It is only when his father passed a comment about his increasing height and muscle mass that Katsuki decided he does not completely hate the changes his body is experiencing.
Before puberty Katsuki had always felt too ashamed to explore his body. During puberty Katsuki examined himself with clinical accuracy like a man possessed. He checks his penis every morning and every night and prays that he sees some growth other than the swelling of his testicles and the sprouting of wiry blonde curls. Midway through the fourth month, he grips his All Might themed ruler in frustration, watches as it explodes it in his hand, completely obliterated. His body is changing rapidly, with one noticeable exception.
Katsuki's hope that public urinals would no longer fill him with dread after puberty dies along with his ruler. He remembers thinking about ways to deal with going to bathroom. Considered just sticking the tip of his dick out through the hole in uniform slacks instead of unabashedly dropping trou and hiding behind some false modesty. It seemed like an alternative to the risk of exposure but the risk of caching his dick in his zipper felt equally terrifying. In the end Katsuki resorted to only ever using private bathrooms and if there were none available, he would grit his teeth and hold it until he could get some privacy.
There were a few uncomfortable weeks when Katsuki was unable to gain access to a private bathroom on a regular basis. A drop in temperature caused a lanky fleshy-eared boy who already struggled to control his water Quirk to flood the boys' bathrooms and the number of private stalls were temporarily drastically reduced. Waiting to use his bathroom at home quickly became the norm during those weeks and the strain of constantly holding his bladder only exacerbated Katsuki's already tenuous control over his aggression.
These were some of the best and worst times. Conflicting feelings of relief, shame and arousal usually accompanied the release of his full bladder. Going after holding it in all day felt satisfying in a way that sent shivers through his spine. It would have felt even better had Katsuki been able to ignore the associated angry frustration that came with the knowledge this situation was entirely due to the shameful secret between his thighs.
The physical symptoms of arousal began appearing with increasing frequency and became more difficult to ignore. As somebody who usually prided himself on his self-control, this development was a major blow to Katsuki's pride.
Whenever he touched himself, below the throb of arousal, lay his shame that he could even deign to think such a tiny thing could give him any pleasure. But it did, oh how it did. For all Katsuki's worries about his shortcomings, his dick functioned normally, even if his release was only a tiny sticky smear.
Getting off became a maddening obsession that clawed at Katsuki's brain until he rutted against his mattress or reached into his boxers to grip his pathetically tiny self between his fingers. To counteract the shame he felt whenever he jerked off, Katsuki would often deliberately dig his nails in, allowing his nails to scratch his dick until he broke the surface of his skin and bled.
As a teenager, Katsuki became fascinated with dicks. Only other people's though. Never his own shameful thing.
Katsuki didn't hate porn. He just…never understood the appeal. The camera spent far too much time focusing on the wrong things. Katsuki had little time for the so called "money shots" or the way the camera lingered on various combinations that made up the tangle of bodies being showcased. Porn with BDSM or extremely niche kinks also filled him largely with indifference. All he wanted to see was the full unobscured length of a thick, veiny cock. He found he particularly enjoyed gifs or clips that showed the gradual hardening of an erection, but clips without a hand or other implement blocking the view were surprisingly hard to come by.
His favourite thing though was seeing other dicks in-person. He would often sneak glances when he and his classmates were changing for gym class. By middle school he had educated himself about how to pack and was consequently no longer ashamed when he was forced to get changed or wear clothing that pulled tight across his crotch. This new-found knowledge allowed Katsuki to use lockers room without devolving into an anxiety attack. Nonetheless, he still avoided public showers and urinals like the plague.
Katsuki's first sexual experience was with a much older boy. Looking back he was probably much too young. He had grudging admiration by the boy's strength-enhancing Quirk and his physique that was reminiscent of All Might.
The older boy had allowed him to push his hand down his pants and give him a rushed, unskilled. Katsuki still remembers the awe he felt at the weight of it. He also remembers the embarrassment he felt when the other boy cupped Katsuki's own crotch and laughed.
"I guess this explains why you were struggling so much with mine. I guess you've never had to use your entire hand before."
Blood had rushed to Katsuki's face and the need to punch someone flowed down into his hands. He felt the sweat gathering and the heat building in his palms. The other boy grabbed his wrist, keeping Katsuki's palms directed away from himself and leant in close,
"You were such a good slut earlier. I'd hate if I'd have to hurt you now."
Katsuki doesn't know if he means he would out him or if he was alluding to how easy it would be to overpower Katsuki with his Quirk. Eventually Katsuki forces his muscles to relax and is released. Even after he had watched the other boy saunter off, the mortification burning under his skin remains.
Katsuki soon found himself craving that combination of humiliation and arousal. He began engaging in increasingly more dangerous sexual exploits as the years went on. His methods of getting his fix becoming darker and more intense.
It never took much more than a wink or a smirk. Katsuki was attractive and even as he got older and filled out, there was always somebody willing to take what he was offering.
Throughout the years Katsuki often found himself on his knees, fully dressed, choking on some nameless extra's fat cock. Katsuki's spit-slicked lips would stretch obscenely as saliva pooled in his mouth and large calloused hands gripped his spiked hair painfully tight. The rougher they were, the quicker Katsuki would cum, his hips grinding fruitlessly against the air as his release stained the inside of his pants.
He liked being down there, liked how these men could put him in his place. Katsuki wasn't stupid, he'd been exposed to locker room talk and popular culture and he understood that despite all the ways he excelled in school and would excel as a hero, in this, he was lower than gravel.