Medea Pideth Machina

Medea Pideth Machina is the overarching antagonist of The Rising of Shield Hero and the true mastermind of the Waves, the darkness of Melromarc, and the actions of the Reincarnators.

In the later part, it was revealed that Princess Malty Melromarc, the series' main antagonist at the time, was a soul fragment to Medea, alongside other villains post-Melromarc who were more or less connected to her somehow. She is the archenemy of Glass and later all heroes in the story once her existence was revealed.

Overview
Medea is a goddess whose goal is to unite the eight worlds which comprise the Heroes World, which have been reduced to two or four by the time the story takes place, and then descend into the fused world to devour its energy and move on, presumably to do the same thing again elsewhere.

She had yet made an apperance in the light novel series, which was adpated from the original light novels, but her existence was already alluded in the recent stories and was referred as a deity/god that was the mastermind of the Waves.

Backstory
With her past being vague to tell, all we know about Medea is that once she was a citizen of an ancient civilization who somehow gained immortality and God-like power. At an unspecific point in time she approached the Heroes' Worlds and started fusing all eight of them into a singe world so she could enter and absorb it.

All the time, she has used her powers to reincarnate and brainwash several individuals into these worlds to sabotage the efforts of the heroes who oppose her while also incarnating her own fragments in positions of authority in said worlds to regulate the process.

Medea had fully aware of her fragments' experience (including Princess Malty Melromarc) in other worlds and shared their memory as a result. She was also the mistress of foes like Kyo Ethnina, Tact Arusahorun Faubley and other Reincarnators. The Reincarnators were forbidden from speaking out Medea's name to their enemies. Once they were about to reveal, they would be literally blew up and killed, and their hand-written cofession and testimony would become too blurry to read.

Personality
Much like her fragments which work to sabotage the heroes and ensure her Waves of World Fusion go uninterrupted, Medea is a rotten, hateful, selfish and remorseless God Wannabe in the core, believing human are just her tools for her to manipulate and admitting everything she did was all for pure enjoyment. She has no attempt of repentance over everything she did. Instead, she boasted her atrocities proudly and had no regards over anyone she trampled underfoot, including Naofumi and later Glass.

Besides, Medea is just as manipulative and deceitful than her fragmental counterpart, Malty, if not even worse. She and her fragments throughout the alternate worlds managed to convince many reincarnated people that she's "good" while sowing discord and death. This act is so convincing that even when she's perpetrating horrific atrocities right in front of them, and confessing to many other crimes, the "heroes" that follow her still think she's the epitome of righteousness.

One of her crueler tactics is to bring back the soul of a deceased enemy (most notably Queen Mirellia Q Melromarc and Elrasla Grilaroc) and send it to their former allies, claiming to have survived their apparent death. After fighting on their side for a short time, the soul will take advantage of an opening to betray them.

In the end, Medea reveals her plans to extinguish all life in the universe one world at a time simply to boost her power level. Before her defeat, Medea suffered from her deflated ego and flied into a rage when beaten and simply trying to take down millions with her. However, this was fortunately prevented as she dies at the hand of Naofumi, who became a God himself and erased everything about Medea from existence, bringing back everyone she controlled as her undead minions.

Ultimately, Medea is nothing but a self-proclaimed sadist who committed her crimes for nothing except greater power, and she always commit atrocities with glee and pride. The villains from the other stories shared her personality, since they were either her incarnations or the same kind of repulsive people she is fond of.

Trivia

 * In Web Novel stories, Medea's existence was only revealed at the beginning of the Final Arc. However, in the Light Novel series, her existence were alluded already after Spirit Tortoise Arc, which had a lot of original contents deviated from the webstory.
 * The author also comfirmed in their words written by the end of Light Novel Volume 19 that Medea (unnamed at the time in Light Novels to prevent spoilers from Web Novels) had more powerful minions to go after Tact, serving as her main generals.