Sir Grodus

Sir Grodus is the main antagonist of Paper Mario: The Thousand-Year Door. He is the leader of the Secret Society of X-Nauts, a villanous extraterrestrial organazation that's bent on world conquest. Grodus' goal was to collect the seven Crystal Stars and use them to break the seal on the Thousand-Year Door, in order to release the Shadow Queen, a 1,000-year old demon who once threw the world into the depths of terror.

His very reason for kidnapping Princess Peach was to use her as a vessel for the Shadow Queen's spirit. After the Shadow Queen possessed Peach, Grodus intended to use her powers to destroy the world, so that he could rebuild it in his own image, and become its ruler.

Paper Mario: The Thousand-Year Door
Like many villains exclusive to the Mario RPGs, Grodus' origin and backstory was never revealed, but sometime prior to the events that took place in Paper Mario: The Thousand-Year Door, Sir Grodus got acquainted with a trio of witches called the Shadow Sirens, which consists of the leader, Beldam, and her younger sisters, Marilyn and Vivian. Grodus ended up enlisting the three of them in his services, and as he did, Beldam and the Shadow Sirens informed him about the Shadow Queen, and how the seven Crystal Stars were needed to break her seal. Upon gaining this knowledge, Grodus wanted to release the Shadow Queen so that he could use her powers to conquer the world. He was then told about the Magical Map that was contained inside of a treasure chest, which had radar-like functions that could track down the location of the Crystal Stars. However, the chest could only be opened by someone who's pure of hearts. Grodus was unable to open the chest himself, due to his evil and tainted ambitions, so he and the Shadow Sirens decided to look for someone who can. That "someone" just-so-happened to be Princess Peach. Not only was she needed to open the treasure chest, but she was needed as a vessel for the Shadow Queen's spirit, as well, since the Shadow Queen could only inhabit a pure maiden.

Grodus sent Beldam to Rogueport, and while she was there, she disguised herself as an old merchant. She soon came across Princess Peach and sold her the map. As soon as Peach opened the chest, Beldam was about to kidnap her, but she didn't, due to the fact that there were many witnesses in the scene. And so, Beldam had no choice but to retreat and return to the X-Naut Fortress. As a result of Beldam's failure to steal the map, Princess Peach ended up mailing the map to Mario, and invited him to go on a treasure hunt with her. However, by the time Mario arrived at Rougeport, she had already gotten herself kidnapped by the X-Nauts. Grodus then scolded and ridiculed Beldam for her failure to steal the map from Princess Peach, because of the fact that Mario ended up getting ahold of it. Beldam tells him not to worry, because all they need to do is take it from him. Somehow, they learned of Mario and company's next destination, which was Boggly Woods, and set off to go steal the map from him. Before then, Grodus ended up giving the Shadow Sirens a sketch, so they'll know what Mario looks like when they come across him. He also sent Lord Crump, his right-hand man, to continue searching for the Crystal Star that they located in Boggly Woods. The Crystal Star was set to be somewhere in the Great Tree, home of the Punies. In the end, by the time Mario and his partners had completed their errand at Boggly Woods, they managed to defeat both the Shadow Sirens and the X-Nauts, defending both the Magical Map and the Crystal Star in the process. After their defeat, they retreated. Grodus was informed of this, and needless to say, wasn't pleased.

By the time the third chapter was completed, an X-Naut researcher, called an "X-Naut PhD", informed Grodus that the seal on the Thousand-Year Door, according to Beldam, was starting to weaken. Grodus, in excitement, begins to talk about how close he is to ruling the world, and that he's planning on destroying the world and remaking it in his own image. He then broke out into a maniacal laughter. Sometime later, Beldam prepared the Superbombomb, a weapon that she prepared that would bring Mario to a "quick and certain end". Although Grodus was displeased with her previous failures and was beginning to lose his patience, he still sent Beldam and her sisters on another mission to stop Mario. This time, their next destination was in Twilight Town.

Super Paper Mario
Grodus himself did not physically or visually appear in Super Paper Mario, but he was, however, given a reference. Francis, the video game and anime-obsessed chameleon has made mention of a TV show by the name of The Grodus Chronicles, which is apparently starring Sir Grodus. It is unknown what it was about or whether or not it had any connection to Starship X-Naut, as no further information was given.

Personality
Sir Grodus is presented as a cold, ruthless, sadistic, and power-hungry villain that's bent on nothing but world domination. He has demonstrated, on many occasions, that he's willing to do whatever it takes to achieve his goals, regardless of how many innocent lives are taken in the process. Grodus is also very serious, uptight and short-tempered, and has shown no form of emotion, other than anger and bitterness, which is mostly directed at his henchmen for failing their tasks. Grodus does not have an ounce of sympathy or compassion for anybody, not even his own associates, seeing a majority of them as nothing more than pathetic inferiors and/or pawns to be manipuated. He has shown as much respect for them as has for his enemies, treating just about everyone around him with as little respect as possible. Grodus also happens to have an extremely domineering personality, and feels that people should always do whatever he says, when he says it, without question. This was shown when he angrily demanded one of his X-Naut soldiers to bring the Shadow Sirens to him, as well as when he attempted to command the Shadow Queen, even after she has shown that she isn't very easily controlled. This, of course, resulted in the Shadow Queen blasting his robotic body into pieces, and nearly killing him in the process.

Grodus has also been shown to be very deranged and psychotic, often breaking out into a strange, unheard-of maniacal laughter after discussing his plans to rule the world. That same laughter was also heard when he used Princess Peach as a shield, and when he torturned Mario and his partners with lightning magic, threatening to murder Peach if they refused to surrender, making him a very sadistic individual. This laugh could also be seen as a reflection of his obsession, and just how desperate he is to rule the world.

While highly psychotic, Sir Grodus has also demonstrated other stereotypical villain traits and characteristics, such as being highly intelligent, and being able to use and manipulate other people. This was shown when he tricked Mario and company into obtaining the last Crystal Star from Lord Crump, and having Doopliss disguise himself as Professor Frankly to trick them into opening the Thousand-Year Door. Grodus also happens to be very skilled at technology, since his fortress, as well as the scepter that he weilds, is constructed with technological advancements that have not been seen anywhere else in the Mushroom Kingdom.

Regardless of the above, the one thing that ultimately led to Grodus' downfall was his desire and lust for power and control. As it turns out, Beldam and the Shadow Sirens ended up taking advantage of this, and ended up tricking him into reviving the Shadow Queen for them. In the end, Grodus was used and manipulated by the very people he thought nothing of, the very people he thought were useless.

Trivia

 * Sir Grodus is the second villain (with Cackletta being the first) other than Bowser to kidnap Princess Peach.