I think I read this online, possibly on Royal Road, and it felt about novel length. The main character is a mechanic at a shop in a world where people have superpowers, which range from extreme durability to superhuman strength to the ability to generate forms of energy like lasers and electricity. One other acknowledged superpower is being able to generate technology beyond the current state of the art. The protagonist does not have any powers, but does have an engineering degree (he's occasionally a bit bitter about how little an engineering degree means in a world where people routinely break the laws of physics by just trying harder). Early in the story, he acquires a set of power armor from an inventor who pawned it. He might have been tasked with fixing it by a local crime lord. Despite lacking any sort of superpowered ability, he's able to tackle the power armor as a regular engineering problem, and solves several inefficiency problems, including bad heat sinks (I want to say that he explicitly mentions that the original design ran a heat sink directly under the groin of the operator, something he notes was practically guaranteed to eliminate the inventor's genetic contribution to humanity) and something involving whether to use gravitic or laser weapons (I don't remember which way he goes).
I think right after the power armor fix, he's approached by that crime lord, specifically by one of the crime lord's two daughters (both beautiful and very deadly) who tasks him with fixing a laser pistol, which he's able to do, again making use of his practical engineering knowledge. Either with the pistol, or with the armor, she basically takes up residence at his house and employment as a guarantee of his completing it and not running, and there's a lot of sexual provocation of him, with him believing that it's because she knows he knows of her reputation for her lovers dying, often killed by her. Her sister has the same reputation. I think there may have been a scene at a party for the crime lord where he specifically calls out one of the sisters for their behavior, pointing out that she's only able to taunt him like this because she knows that no one is willing to confront her on her behavior, something that he later learns actually hurt her personally (I think maybe with an indication that she was not behind their deaths, but rather that it was due to her life as a member of a criminal family). One of the sisters wielded electricity and the other had superpowers, although I don't recall what. I think their father wielded laser blades. The sisters had different mothers. He winds up in a sexual, if not romantic, relationship with one or both sisters through the course of the story.
Somewhere in the middle of the armor development, the shop gets attacked, and he's forced to enter the armor while it's half built, and is able to fend off the attackers with the help of one of the sisters, which somewhat cements him as not only the mechanic, but also the pilot, of the suit. I think there may have been some element of him having used software that adapted to the pilot, and by jumping in the suit, he essentially necessitated rebuilding the software if someone else becomes the pilot. In a later episode, the original owner of the armor shows up, wanting it back, and he realizes that the inventor either hadn't developed proper heat-shielding, or still hadn't fixed the heat sink problem, which resulted in him improvising a molotov cocktail from the bar and burning the inventor to death by overwhelming the shield.
Later in the story, the crime lord is on the ropes with his men being killed off in incidents which suggest insider knowledge, leading to internal purges that further destabilize the organization. Eventually, the protagonist winds up buried in a wine cellar after an explosion and confronts the enemy, who is a psychic, something formerly unknown in this world. I want to say that he's dispatched by some sort of remote explosive trap set by the protagonist, and just barely triggered by force of will breaking him out of the psychic's grasp. I'm fairly sure that the crime lord that hired him was killed in that attack.
At some point in the story, it's revealed that the protagonist is operating under an assumed identity, the result of being part of a family of superheroes, including one of the more powerful ones in the setting, his sister. I think she pays him a visit at some point, offering him better work than what he's getting in the shop (and implying that it would let him stop working with criminals), and at another point the crime boss makes it clear that he's aware of the protagonist's true background (but, I think, not specifying the details) and is holding that over his head.
I don't remember exactly when I read this, probably within the last five years. I think I later learned that there were sequels, but that only the first book was available for free.
