A soldier from another galaxy, away from Popstar. Part of a group called the Star Warriors, a mercenary army group mainly composed of people descended from the Aeon Warriors (Galacta Knight's species). During Meta's time with the Star Warriors, he mainly worked fighting against Nightmare's demon beasts. Due to his time fighting them, he finds them weird, off-putting, and dangerous instinctively. He also doubted their intelligence until he found ones that were indistinguishable from real people in Dreamland.
Another large part of the Star Warriors is preventing Dark Matter from expanding their domain across the universe, particularly by seeking out anti-DM weapons that are across the galaxies, mainly left behind by ancients, though other naturally occurring ones exist. Meta's Galaxia sword originated from one of these missions, and Meta was able to keep it despite not being particularly high up, because it wouldn't accept anyone else using it. Since he was stuck with an anti-DM weapon, he ended up getting assigned to missions directly attacking DM, though mainly smaller ones, investigating potential issues and stopping small groups of lesser DM. Even these missions were dangerous though, so he grew an appreciation for how dangerous and invasive DM are extremely quickly.
Coincidentally (?), sometime later, Meta's home planet is attacked and destroyed in a DM attack, and the battle also wiped out many people Meta worked alongside in the Star Warriors. He was unable to cope with the PTSD that left him with, so he ended up defecting from the army and sought out the most quiet, backwaters place he could find to hide and retire at. This led to him coming across Dreamland, and Dedede was more than happy to hire an intergalatic soldier as his head of army.
After everything he went through, Meta is a particularly paranoid person, and he distrusts a lot of the people who end up showing up in Dreamland. Gooey, Marx, and Magolor get the brunt of this over the years. A particular example of his PTSD is that, after the events of KDL2 and he found out Dark Matter Swordsman was possessing Dedede, he realized Dreamland is a very weak planet and would fall to an attack immediately, which resulted in the Revenge of Meta Knight story. Getting defeated there resulted in Meta fleeing to avoid a repeat... but... he came to realize that DM wasn't advancing in the region, and when he returned, he learned that the two Zeroes had been defeated. Seeing this, he decided to stay in Dreamland and help train Kirby further, figuring she must somehow be connected to something that will bring down Dark Matter once and for all.
As a side note, Dark Matter Swordsman was made as pretty much a mockery of the Star Warriors, as they are space knights, and DMS is a DM knight. DM usually don't use physical weapons, or have such appearances, and DMS was newly created. This blatant taunting was not lost on Meta.
Another note is that Meta is perceived as being an extremely strict, professional, and intelligent person by many Dreamlanders. And while he is those things to some extent, it's not nearly to the degree people think. Within the army, he was a bit of a slacker... he took combat seriously, but skipped out on studies, and wasn't above finding ways to get out of casual tasks.
Nightmare, like his anime counterpart, runs the NME business creating and selling demon beasts of all sorts. Creating them both to sell to others and for his own uses, he causes chaos and wars across the galaxies. Dedede, before the main events of the series start, was buying demon beasts from NME... mostly out of boredom, but also to get servants like Lolo & Lala, as he struggled to find enough capable people around to do the work he wanted. Eventually, Nightmare came to Dreamland as Dedede was not paying his debts, but Dedede was instead the one who captured Nightmare (in the Star Rod). He is destroyed in the events of Kirby's Adventure.
Marx was created by Nightmare to be his temporary replacement if anything happened to him. But it's only intended to be temporary, as Marx is meant to seek out a way to revive Nightmare as quickly as possible. Knowing Galactic Nova could grant one wish, he sought out Kirby to figure out the way to activate it for him. He figured Kirby should do it because hey, she killed Nightmare, she must know what she's doing.
Once Galactic Nova was activated though, Marx went a bit mad with power, and decided to wish to take over the universe instead. If he controls the universe, he can revive Nightmare easily, so it's still fine! ...Sometime after being defeated by Kirby, he revived, and decided that he doesn't actually care about reviving Nightmare or getting revenge or anything like that. And so, he takes over as being the NME head, though he's just a scammer, as he can't actually make demon beasts, nor does he care about creating an empire.
"Admins" are anyone that are below Nil/Void Termina, but above normal Dark Matter. Generally they either have leadership, or are strong enough to lead/take control of attacks. The two Zeroes are the top admins, with Zero being the offensive leader and Zero2 being the defensive leader. Zero is powerful and terrifying to non-DM and DM alike, but his main weakness is folding and making rash decisions under stress or anger. Zero2's focus is ensuring the number of DM stays healthy and reorganizing when they're not. She has a bad sadistic streak.
Miracle Matter is the main admin under Zero2, utilizing her seeming invincibility to keep anyone from making too aggressive of attacks towards the Zeroes. As her only weakness is Copy Ability, and Copy Ability is a DM ability, she can only really be hurt by other DM. She is much more like typical DM, as she doesn't appear to have any emotions or thoughts of her own, blindly sticking to whatever Zero2 tells her to do. In a way, she is a replacement of Nil, having many of her abilities, though much weaker and unable to take advantage of them without the brains to back it up.
Star Warriors (and others who fought DM) would know about the Zeroes but not really have any concept of what they're like. So instead, they would severely fear Miracle Matter, as her arrival at a fight would be a sign to retreat or die.
Dark Matter Swordsman (DMS) and Gooey were the latest created admins. DMS' appearance and sword ability likely spawned from Zero's hatred of the Star Warriors and wanting to create something that taunts them on equal footing. DMS is particularly good at possession and stealth, rather than brute force, but it has the typical DM abilities of only being weak to certain weapons and so on. He got a bit of use on the battlefield until he was sent to Dreamland. After getting beat up with the Rainbow Sword, he was alive but weakened and alone, and in this time, realized he really doesn't have to listen to what Zero says.
...It should be noted that DM are supposed to be created emotionless and without free will, but obviously this isn't always the case. After being created, new DM are essentially ritualistically lobotomized, which results in the loss of one eye. This ensures they don't have free will, and are properly connected to the DM hivemind. Sometimes, it doesn't work out properly. This is why DMS is able to come to certain realizations himself.
In Gooey's case, he was able to avoid the ritual entirely, through some trickery and luck (Zero was not around at the time to do it himself). He still pretended he had it done, because he didn't know what else to do, but he got caught by Zero eventually. Zero, enraged at such treachery (and deep down, a bit concerned at what else this guy could pull off), basically beat him near to death and tossed him out of Hyper Zone. Unfortunately for Zero, he didn't properly check if he was actually dead, and Gooey ended up in Dreamland alive. Gooey is stressed about the possibility of being re-captured and used in the future, but this doesn't stop him from helping fight against DM, first helping Kirby put together the Rainbow Sword, and then directly confronting Zero in DL3.
Dark Nebula was an admin once upon a time, but was locked away for so long everyone kinda forgot about it and it became severely weak... So it's not really important... sorry fella...
For the record, Zero did really regret "killing" Gooey after his temper wore off. DM were already losing numbers, and having a strong and powerful admin would have been very useful. If only he restrained himself and just completed the ritual, he'd have a great admin to control...
Bonus fact: Gooey's Mandarin name is 蠱依 (Guyi), which means roughly "relies on dark magic" or "obeys dark magic". With 蠱 meaning bewitching, malevolent magic. It's similar to a term for possession too. Just thought that's neat!