The Mario franchise. For the first couple of entries in the series, all of the characters were unlocked from the beginning, and in the most recent entry, Mario Kart 8, characters were unlocked in a random order as players played.
However, for several games in the series, collecting every character was a challenge, requiring players to take on some of the games' hardest tasks, like completing difficult races on different engine classes and taking on staff time trials, in order to unlock series-favorite characters. Players who are fans of these characters will have to buckle in if they want to play as their favorites, but the work is worth the reward.
Metal Mario - Mario Kart 7
This shiny metallic version of the platforming plumber first appeared as a playable racer in Mario Kart 7, where players had to get first place in the 150cc Special Cup in order to unlock him.
The Special Cup in this 3DS entry, as usual, includes versions of the classic Bowser's Castle and Rainbow Road tracks. However, even if players can conquer those two difficult tracks, they will need to take on the icy caverns of Rosalina's Ice World, as well as the tight turns and crazy jumps of DK Jungle.
Waluigi - Mario Kart DS
As the game that introduced retro cups, Mario Kart DS wasted no time in utilizing classic tracks in the game's character unlock criteria. Fan favorite and Smash Bros. Waluigi is one such character, requiring the player to win every retro cup on the 100cc engine class.
Although this engine class is not the most difficult in the game, winning all of the cups will still present a challenge, as some tough classic courses have returned for this game. If players can survive the tight turns of Yoshi Circuit and the ghoulish treachery of Banshee Boardwalk, though, Waluigi will be theirs.
Lakitu - Mario Kart 7
Normally, Lakitu can be found as a part of the crew running the races, but beginning in the 3DS title Mario Kart 7, the cloud-riding koopa became a playable racer. In order to add him to their roster, though, players have to win the Lightning Cup on the 150cc engine class.
The Lightning Cup is the toughest retro cup in the game, asking players to make their way through challenging, obstacle-filled courses like Dino Dino Jungle and Airship Fortress. The toughest of them all, though, may be the SNES Rainbow Road with its many sharp turns and precarious corners standing between players and Lakitu.
Petey Piranha & King Boo - Mario Kart Double Dash!!
This dynamic duo of mischievous villains are notable in Mario Kart: Double Dash!! for their ability to be able to use the unique item of any other racing pair in the game. They're also notable for being fairly difficult to unlock, as they will be the last characters players get in the game.
In order to unlock Petey and the King, players will need to win the game's Star Cup in Mirror Mode, taking on mirrored versions of challenging courses like the traffic-filled Mushroom City or the precarious DK Mountain. Petey Piranha, in fact, never returned for any other game in the series, making this challenging process the only way to race as the forgotten character.
Rosalina - Mario Kart Wii
This Super Mario Galaxy fan favorite was hugely popular at the time of Mario Kart Wii's launch, but players who wanted to race as her may have had trouble unlocking her. Rosalina requires players to achieve at least a one-star rank on every cup in Mirror Mode, the toughest engine class in the game.
Not all hope is lost for any players who just can't get the hang of the mirrored tracks, though, because Rosalina can also be unlocked by completing 4,950 races total. If players are using a Wii console that has a Super Mario Galaxy save file, however, that number is reduced to just 50.
Dry Bowser - Mario Kart Wii
Bowser's skeletal variant appears as a kart racer for the first time in Mario Kart Wii, but players will need to work if they want to play as this bony heavyweight. The primary method to unlock Dry Bowser is to get at least a one-star rank on every Wii Grand Prix cup in the 150cc engine class.
This presents quite the challenge for players, as they'll have to take on several tough courses against some of the toughest opponents in the game to add Dry Bowser to the roster. However, players can also unlock him by playing a total of 4,350 races, so anybody having trouble with unlocking Dry Bowser should be able to snag him eventually.
Funky Kong - Mario Kart Wii
This cool-as-a-cucumber surfer from the Donkey Kong franchise made his kart racing debut in Mario Kart Wii, where he quickly became a fan favorite for his high speed. However, new players may have trouble getting to play as this bandana-clad racer.
Players can unlock Funky Kong by unlocking four expert staff ghosts in the game's time trials. This may not sound like much, but beating normal staff ghosts to get those expert levels is no joke. Alternatively, players can play a total of 2,250 races or win 25 WFC ghost races to add Funky to the roster, both tasks that may take some time on their own.
R.O.B. - Mario Kart DS
The classic NES peripheral appeared as the final unlockable character in Mario Kart DS, although he has yet to return for any other entries in the franchise. The cute little robot did make his mark on players of Mario Kart DS for his challenging requirements, though.
In order to unlock R.O.B., players must get a gold trophy across either every nitro (new tracks) or retro (classic tracks) cup in the game in every engine class. This means that players will have to come in first across every cup in 50cc, 100cc, 150cc, and Mirror Mode, making for quite the challenge, especially for new players.
Baby Luigi - Mario Kart Wii
While his other baby companions are either unlocked from the beginning or have fairly easy unlock criteria, the adorable Baby Luigi is actually one of the tougher characters to get in Mario Kart Wii.
Players will have to beat eight normal staff ghosts in the game's time trials mode satisfactorily enough to unlock the expert staff ghosts for those eight tracks. While there are 32 total to take on, it still might be challenging to beat enough of these times to add the tiny driver to the game's roster.
Mii Outfit B - Mario Kart Wii
The most difficult character to earn in Mario Kart Wii is this alternate outfit for the player's Mii characters, allowing players to play as themselves and their friends in outfits resembling Mario and Luigi's overalls or Peach and Daisy's princess dresses.
In order to get these new duds for their Miis, players have to unlock every single expert staff ghost in the games' time trials, meaning they must beat all 32 normal staff ghosts by a certain margin, depending on the track. Alternately, players can unlock this outfit by participating in 5,100 total races or winning 5,000 WFC ghost races, neither of which are easy tasks in themselves.