After beating killer7 once on any difficulty, you can go back and re-play the game in killer8 mode, which gives you access to the previously unavailable Young Harman character, whose huge amount of health and tommy-gun make him almost a Game Breaker. You'll need him though, since the only difficulty level available for killer8 mode is Bloodbath, where enemies take obtuse amounts of damage, give almost no blood at all, and have invisible weak points.
Street Fighter Alpha also had Akuma, but joke character Dan Hibiki made his debut as a secret character in the first Alpha game. Alpha 2 featured Shin Akuma and Evil Ryu.
SNK vs. Capcom: SVC Chaos had a buttload of secret characters. Some of them were the ones you woudl expect like Dan or Geese, while others included Zero and Demitri. It also featured an evil version of Ken, called Violent Ken, based off of his possessed self in Street Fighter II: The Animated Movie.
Battle Arena Toshinden: In order to play as Gaia and Sho, the two bosses, you had to enter two codes on the title screen before the menu showed up, then select either Eiji or Kayin while holding certain down certain buttons on the D-pad
Most of the modern WWE games have former WWE talent as hidden legends.
Recent WWE games can be hacked to unlock WWE management like Vince McMahon, Jim Ross, Michael Cole, Lillian Garcia, Mike Chioda and Nick Patrick.
Typically, you first unlock Jigglypuff by beating single-player mode once. Another standout secret character is Luigi, the only other character besides Jigglypuff to have been a secret character in the first three Smash games. Notably, Brawl mixed it up by making Jigglypuff one of the last characters you unlock.
For other characters, the requirements vary according to the case. Each time you meet the requirements for any character, you will fight him/her/it with whichever one you were playing as at the time, and if you win you will unlock the character. Other typical requirements include beating the game without continues, with a specific character, or completing certain side events. Each game would also give you the characters if you logged a certain amount of multiplayer hours. In Brawl, there are three ways to unlock each character: getting them to join you in Adventure Mode, playing a certain number of Vs. matches, or by doing other events.
Mewtwo in particular takes the Guide Dang It to the extreme, you have to play over 20 player-hours (that is, 20 hours divided by the number of people playing) or fight around 700 matches.
Tekken's "Devil" was unlocked by leaving the game on for 30 hours. Alternatively, one could get a perfect score in the game of Galaga'' that replaced a standard Loading Screen at the very beginning of the game.
In the Killer Instinct games, the Big Bad could be played as by entering certain button combinations as the fights loaded.
In the fighting game One Must Fall, it was possible, through a series of button sequences, to play as both the end boss, and use his towering robot (not normally available in the story mode). There were also a lot of secret characters you couldn't play as, but could access for special fights, both in story mode and tournament play—"unranked challengers". In tournament mode, some of these unranked challengers would appear in other tournaments, some were the only appearance of a story mode character in the tournaments, some were references to other Epic Games franchises, and some were just unique characters who only popped out surprise you.
Guilty Gear is outright vicious about unlocking characters. In XX, to unlock Robo-Ky, Kliff, and Justice, you must do one of the following: complete forty of the game's fifty (insanely hard) missions; complete all the story paths (some of which are Guide Dang It, some of which are outright painful, and some of which are both) for all characters; or complete over 200 levels of Survival Mode (which amounts to seventy or so battles - including twenty much harder "Daredevils"). Or just leave the game running for several days. Then, there are the laternate movesets for most of the characters, which are practically characters unto themselves...
Marvel vs. Capcom: The Marvel secret characters are different palette swaps of the Hulk (gives him his Marvel Super Heroes movelist and properties), War Machine (has incredible defense and armor, now fires missiles instead of lasers) and Venom (becomes the fastest and weakest character in the game), while the Capcom side has Shadow Lady (Chun-Li transformed into a cyborg ala Shadow), Lilith-Mode Morrigan (Morrigan with Lilith's color scheme and properties) and Roll (a Joke Character who has Megaman's movelist).
Tatsunoko vs. Capcom: In the original version (Cross Generation of Heroes), Saki Omakane, Hakushon Daimao, Ippatsuman and Viewtiful Joe needed to be unlocked by beating the game a certain number of times with either a Tatsunoko or Capcom character, and then purchased in the store. In the updated version Ultimate All-Stars, the above four (save Daimao for licensing reasons) were made into starting characters. Now, Tekkaman Blade, Joe The Condor, Frank West, and Zero are unlocked only by beating the game with a set number of characters from both sides. Beating the game with Blade, Joe, Frank and Zero would unlock Yatterman-2.
Mortal Kombat is basically the trailblazer for secret characters in fighting games. For the first two games, they were simply unplayable, but far stronger and faster palette swaps of the ninja characters. Starting with MK 3, they could be unlocked.
First up is a roboticized version of Smoke, the first ever unlockable character in the series which you could unlock by entering the "Ultimate Kombat Kode", which used a total of ten slots, divided up by inputs from both players.
In the update following that, Ultimate Mortal Kombat 3 adds three new unlockables each with their own UK Ks which includes "Classic Sub-Zero", Mileena and Ermac. Smoke is relegated to a normal playable character, but by inputting a code before any round, he will change into his much faster human form, which is basically a faster silver colored swap of Scorpion.
The original arcade version of Mortal Kombat 4 had a rare player-exclusive version in Meat who used the skinned corpse model utilized in a couple of Fatalities but used other characters' moves instead and could be unlocked.
The console versions add Goro and Noob Saibot, both of whom were playable by selecting Shinnok or Reiko respectively while using the Hidden option. For some reason, Reptile became this in the Game Boy Color version.
MK Gold added Sektor to the mix by doing the same with Cyrax.
Many of the characters in Deadly Alliance could be unlocked by purchasing their Koffins, which had both a specific koin count and type of koin but two in particular were only available after clearing everyone's Konquests; Blaze and Mokap. However, neither have a weapon style or Fatality.
Deception continues this but the characters and costumes now require a key to open. There's an especially infamous example in the case of a zombified Liu Kang whose key could only be found a very specific date and time and in an obscure location. The payoff is worth it since he actually deals twice the damage against Onaga. You also technically play as one through the entirety of the game's adventure mode until you cleared the main story where he will be available in all modes but the size of his moveset, which is taken from most, if not the entire cast, is dependent on how many keys you can unlock, including his Fatalities.
There are a few secret characters in the Sonic the Hedgehog series. The Super Mode versions might count, but there's also Metal Sonic in Sonic Adventure DX, who is available after getting all 130 emblems (though he's only available in trial mode and can't do Sky Chase), as well as Amy in Sonic Advance 2 after beating the game with all the Chaos Emeralds with the previous 4 characters.
The first Viewtiful Joe game lets you play as Sylvia, Alastor, Captain Blue and even Dante (who, unfortunately, is only unlockable in the PlayStation 2 version of the game) by beating the game's multiple difficulty levels.
The PS2 version also lets you use all the characters (including the PS2-exclusive one) with a button code.
Super Puzzle Fighter II Turbo: Scrolling off to the side on the character select screen revealed a few secret characters, including Dan. Also, holding down a button while selecting certain characters turned them into others.
Wrecking Crew '98 had four original secret characters: Onigiri, Onnanoko, Oyaji and Dogu. As with the normal opponents, Defeat Means Playable, but they might not even be seen on a normal playthrough.
Harvest Moon Frantic Farming lets you unlock the Witch Princess after completing the rest of the stories.
Real Time Strategy
In Warcraft 3, you can recruit a Hydralisk unit from Starcraft for one mission. It was only one mission, but he was hidden quite a ways away.
Likewise, in the expansion pack it was possible to receive an extra hero unit for one mission providing you first triggered a secret mission in the preceding mission and then completed said secret mission.
In racing games, you often have secret vehicles rather than secret ''characters''. Though some games like the Mario Kart series are character driven enough to have actual unlockable characters. In the case of Mario Kart Wii, not only you can unlock characters by acing the Grand Prix Cups, but also by unlocking ghost racers (not playable, they're simply leftovers of previous races) in Time Trial, for which your times have to be exceptionally good.
The Cheer Girls and the Divas in Ouendan and Elite Beat Agents aren't particularly secret; they have their own bios in the manuals and everything, they're just really, really hard to unlock.
Not terribly. You get them by beating hard mode, which unlocks Harder Than Hard mode. in EBA< beating that allows you to play as Commander Kahn.
Space Channel 5 Part 2 somewhat does this. By completing Reports with 100% and by doing other things, you can play as other characters in the game, including the Big Bad.
Certain demon-lords in NetHack have no "fixed" lair and only turn up when/if summoned. It is entirely possible to finish the game and never meet them. (And in the case of Demogorgon, this is mostly considered a good thing...)
Wild ARMs: In the second game, you recruit Marivel in a completely optional dungeon. Alter Code, the remake of the first game lets you recruit Zed after a Bonus Boss fight and after you give him an item acquired in the last dungeon.
Suikoden II lets you recruit the overpowered main character of the first game as a non-Star. It required a last dungeon save file from the first Suikoden and a Side Quest.
Planescape: Torment: You can get a special character, the Rogue Modron Nordom, only by exploring a bizarre pocket dimension where the inhabitants are trying to understand the concepts of dungeon-delving. The result is a randomly generated maze, creating by opening and shutting various doors between the myriad chambers, and Nordom can only be found in a specific chamber. To even get to the pocket plane, you buy a specific item (the "Metallic Cube" - a poseable Modron figurine) from Vrichika's Curiosity Shoppe. Then, after talking to the Modrons in the Brothel of Slaking Intellectual Lusts to discover that it is a portal, you have to figure out on your own the specific pattern you need to alter the figure's limbs into to activate it (extend left wing, extend right wing, rotate right arm). Once there, you first have to fight your way through a randomly generated maze (fortunately set on "Easy" difficulty) to find the control room. Then, from there, you have to set the maze to "Hard" and then try and find the room where Nordom is waiting. Did we mention that, on this difficulty, the maze is filled with powerful enemy constructs, which can make this a Bonus Level Of Hell if the party doesn't time Level Grinding on either the Easy or Medium settings? And that, as this maze is randomly generated, you may need to reset it over and over before Nordom's chamber becomes accessible?
However, all Torment PC's are unique and don't even stand out very much from the crowd given that unique characters are all over the place. As with many other such characters, Nordom is hinted at but not explicitly identified. Morte and Annah are the only characters you will definitely encounter. Second, Nordom is also always found in his dungeon on Hard if you look around. Third, let's not forget Vhailor, who is even more secretive: you have to walk past a certain portal into a room you couldn't get into before, and which doesn't obviously show up.
In Chrono Trigger, near the end of the game, you are presented a choice. Take one path and you can recruit Magus. Take the other and the mystery character is Lost Forever.
Breath of Fire II: Bleu/Deis. You must walk over a specific panel in a random desert with nothing else in it after reading a certain book. The book both unlocks the panel and tells you, if obtusely, which one it is.
Final Fantasy VI: Gogo the Mime was found in a small optional dungeon which you accessed by fighting an enemy that only appeared in one place, and letting it eat you instead of killing it.
Also from Final Fantasy VI was Umaro who can be found by jumping off a cliff where the Esper Tritoch/Valigarmanda was and having Mog in your party.
Final Fantasy VII: Yuffie appeared as a random battle, and the player must select certain dialogue options to get her to join. The game was merciful, though, in that she would appear later until the player gets it right (justified as she stole from you each time you got the sequence wrong). Also Vincent, who involved solving puzzles in a haunted house. After finally finishing the puzzles, you has to fight a boss. The boss was very tough if you didn't know what you were doing, and there's still little indication on what you're supposed to do to actually recruit Vincent.
Time Stalkers has Marion. Although not a secret by any stretch (she's listed in the manual with all the other characters), the method for getting her is convoluted and relies entirely on luck—you have to find her as a randomly generated item.
In the Pokémon games, there are a few secret characters in every game. First generation had Mew, second generation had Celebi, third generation had Jirachi and Deoxys (as well as Mew, but the required item to access it was never released outside Japan and fourth generation had Darkrai, Shaymin and Arceus. Then again, Arceus being the equivalent of God in the Pokemon games makes it so he better be a secret character, or else the game would probably break.
To any Americans playing Gen. II: No Celebi, sorry. The related event series was removed for the English version.
In the Gen II remakes,Celebi is an event downloadable. And there's the Spiky-Eared Pichu, plus the secret Bonus Boss battle with Giovanni.
Natsume in .hack//G.U. Vol. 3: Redemption. She only appears if you've defeated all the Chaotic Player-killers, and even then, you can't actually get her in the party until after you beat the final boss. Tabby from Roots is similar, as you find her on the 50th floor of the game's Bonus Dungeon.
With high enough Paragon/Renegade skill in Mass Effect 2, it's possible to kill Samara during her loyalty mission and replace her with her daughter Morinth.
Futomimi and Sakahagi in Shin Megami Tensei III: Nocturne. In Hell's Maze, you can encounter two souls; one that's red (Sakahagi) and another that's green (Futomimi). Talking to both of them causes a corpse to show up in Zoshigaya Cemetery, which is holding the Afterlife Bell. Bring the Bell to the two souls and ring it to bring them Back from the Dead as their former selves. That alone won't unlock them, however; you still need to fuse a pair of demons to get them. Dante and Black Frost also count, but they join you instantly regardless of your level.
Speaking of Megaten, from Persona we have Reiji, who is infamous for this. From Persona 2, we have the Custom Personae, which can be unlocked by certain dialogue choices, and Durga, which needs a certain choice as well. Also, from Eternal Punishment, the Ancestral Personae and Lugh.
The HellfireExpansion Pack for the original Diablo came with two secret, "unlockable" characters: the Bard and Barbarian. The Barbarian has since reappeared in the sequels.
In Arcanum, while not many characters are hidden per se, most people will never know that certain NPCs are can actually be recruited, mainly because some require special dialogue options and/or quests to be accomplished before they even consider joining.
Another character, called Dog, requires that you find him within a time limit as soon as you enter the town he's found in.
Shoot Em Up
Star Fox Assault 's Multiplayer mode lets you play as the core Star Fox team, but Wolf O' Donnell can be unlocked. He has the same total stats as Fox, but with three maxed out skills (a maxed out speed in particular that lets him run around all the other characters), he is considered by some to be a Game Breaker.
Raiden Fighters Jet has the Slave and Fairy planes, unlockable by holding the bomb button when pressing Start at the title screen. The Slave will replace whichever plane you select, inheriting its movement speed and bomb; the Miclus is replaced by the Fairy instead.
In Phantasmagoria of Flower View you can unlock Merlin and Lunasa Prismriver if you complete the Nintendo Hard Extra Mode with Komachi and Eiki Shiki. Given that this game is from Bullet Hell genre, many casual players won't even try this (it's like the story mode, but with number of bullets on Lunatic standards, both you and opponent having one hit point while she has the invincibility time for the beginning of the fight, and without any spare lives or continues).
In the iOS port of DoDonPachi Daifukkatsu you can unlock Hibachi as a playable ship. It is just as powerful as its boss counterpart.
This is a fixture of the Tokimeki Memorial series. Each game contains at least two of them :
Tokimeki Memorial Girl's Side : Aoki Chiharu, Tendou Jin (for the DS Version 1st Love)
Tokimeki Memorial Girl's Side 2nd Kiss : Akagi Kazuyuki, Majima Tarou and Komori Taku (for the DS Version 2nd Season)
In Frozen Essence, not completing the requirements for any character's path unlocks Zareh's path.
Lucky Rabbit Reflex, a Dating Sim heavily inspired by the aforementioned Tokimeki Memorial, has Perth (who can be met only by attending a certain club and picking specific choices) and Mardi (who, like many Tokimeki Memorial secret characters, requires a lot of shopping trips to unlock his route).
The Backyard Baseball series has secret characters ranging from Mr. Clanky to the announcers to freaking Babe Ruth.
Super Robot Wars: Many secret mechs in the Super Robot Wars series. The methods for getting them vary from the simple (kill X number of enemies with character Y, especially easy when character Y is a walking engine of death) to the Guide Dang It, (beating a certain enemy with a single character, not attacking a particular unit in a fight, or reassigning a character to an incredibly weak robot before a certain mission, or winning a Hopeless Boss Fight).
And quite often, there will be two or more hidden mecha/weapons that cannot be gained in the same playthrough (ex: Hyperion Gundam and Tekkaman Rapier in W, which are achieved on different sides of the same route split), making 100% Completion impossible.
The PSP port/upgrade of Final Fantasy Tactics, Final Fantasy Tactics: The War of Lions, included Cloud and all of FFT's unlockable characters, as well as Final Fantasy XII's Balthier (as a secret character) and Final Fantasy Tactics A2's Luso (met in the story).
Final Fantasy Tactics Advance: 4 characters offer to join your party if leaving with certain mission items, 4 others from completing certain missions.
Final Fantasy Tactics A2 had Frimedla Lotice, gained from doing five separate quests. The game also had Vaan, Penelo, Al-Cid Margrace and Montblanc as optional party members.
Shining Force was horrible with these, and it extended to the sequels.
The game Vantage Master by Falcom allowed you to play as characters from their other games, by answering the questions to get a specific class, then naming the character appropriately. For instance, becoming a knight and naming him "Adol" lets you play the knight from the Ys games, while naming the swordsman "Mail" gives you the title heroine from the game Popful Mail.
Fire Emblem: Path of Radiance has Stefan, who is very, very vaguely hinted at... and nowhere in the hint is it mentioned that you can only unlock him with two specific characters (neither of which are the main character, whom you use to recruit about 85% of the recruitable units). He's very much worthwhile, and would be even if he didn't come with the second best sword in the game, losing only to the Eleventh Hour Superpower sword Ragnell (which only Ike can wield anyway).
Radiant Dawn is far worse. Stefan returns as even more of a Guide Dang It, especially as you only get your vague hint after he's Lost Forever. But even worse is the other Secret Character, Sephiran a.k.a. Lehran. You have to go through several convoluted steps (hinted at nowhere in the game), one of which can be completely impossible if you were unaware of it several chapters beforehand (or even just had bad luck with the Random Number God). And if you finally do unlock him, guess what? You get him for one chapter. Namely the final boss battle. In which he comes with no weapon, meaning you just went through all those steps for practically nothing. Admittedly he does make a good white mage, but you should already have two of those (and possibly more).
If you're willing to give up Micaiah or Pelleas(if you got him) for magical attackers and give LehranRexaura or Balberith, turns out, he's actually fast enough to double attack the Final Boss. For added fun, and possible symbolism, have him team up with Micaiah and perhaps Sanaki to bring Asherah down. Nothing quite like happy god-slaying family bonding time, right?
Advance Wars had Nell, and the sequel added Hachi. Both are gamebreakers.