wario
Wario is a greedy, modern pirate with a strong sense of capital investment. He steals without care, fights with his bare fists, and sells his wares at triple their price. He is a menace to both Kingdoms, and everyone hopes for the day when he and his brother and put behind iron bars. Permanently. That being said, he is not without his merits. He steals mostly from those who own too much for their own good, generally only fights those too cowardly to stand up against the unfair oppression of the ruling class, and he brings rare goods to those who would never get them otherwise. He lives in a very unusual balance between villain and hero, and he sure does make a profit while doing so. He and his brother appeared after Mario and Luigi came to the Mushroom Lands, and he named himself Wario and his bruther Waluigi as a mockery of the "blindly obedient morons" (his words).
waluigi
Waluigi goes wherever his brother goes, but he is much less genial in nature. While being a pirate and feeding the poor is a nice idea, he doesn't really care for it. No, he wants the gold, the castles, and the power that the ruling class enjoys for himself. Of course to get those things he would pillage and rampage throughout the lands without mercy, but his big brother has kept Waluigi in check over the years. He is simply biding his time until the day he has enough money to fund his own operations and start building an evil empire from the ground up. He particularly hates Luigi with a white-hot passion, for the green hero's relationship to the beautiful and resilient Princess Daisy has made him mad with envy. He'll take any chance to make Luigi plummet to a dark abyss if it means he can finally woo Princess Daisy as he has always wished.
king wart
King Wart is another subspecies of the Koopas, but a very, very distant one. Instead of having an officially recognized Kingdom with a territory, King Wart dwells underground in caves and sewer systems. He only has a few people loyal to his cause, which is to take revenge upon the surface world for treating the underground so poorly (littering it with refuse and toxic water). He one time concocted a hallucinogen that put the victim into a dreaming state while still awake. He infected the entire Mushroom Kingdom and manipulated them into thinking he was their rightful ruler. Only the Mario Brothers, Princess Peach, and Toad retained their willpower, and so the four quested together against the very people they were trying to save in order to "overthrow" King Wart. In the end they succeeded, and King Wart escaped back to his hidden subterranean world. He still dwells there today, but he is always scheming up more plots to return to the Mushroom Kingdom.
donkey kong
Donkey Kong was one of the original villains in the Mushroom Lands, and it wasn't until the Mario Brothers defeated him that he began to reevaluate his life. He realized that all he was doing was bringing harm upon his own family, and when he finally gave up world domination, his enemies saw him as weak and began to attack him in force. Over the years he has triumphed over many enemies in the quest to keep his family safe, and he has been lucky enough to be able to rely on his family for help too. He has been acquitted of his criminal history by Princess Peach, and he is eternally thankful for his second chance at life. He still refuses to travel outside of his lands, even if it means to aid an ally. He fears for his own family too much.
wendy
Wendy was a fashion model of sorts for the Mushroom Kingdom, traveling across the lands and appearing alongside both the Toadstools and the Koopas in new clotheslines. She was considered to be the most beautiful woman in the world and also the least obtainable one. Suitor after suitor would come for her hand in marriage, but she refused them all. It wasn't until she saw a rampaging Donkey Kong that her heart knew its other half. She approached the marauding beast and begged him to take her along. He loved her just the same, and he took her with him then continued his rampage. The public assumed the wrong idea and claimed Donkey Kong kidnapped her. It wasn't until after the beast was beaten and Wendy decided to stay that everyone realized it was true love. She still lives with him today, and they are quite happy together.