Promoted to Playable

This is when a character who was computer-controlled (as an NPC or as a Boss) in one installment of a series becomes a potential player-controlled character through the course of the game, or in sequels. They may be hidden unlockable characters, you might get the chance to recruit them, or they might be presented outright at the character select screen.

Typically they are more powerful than your average "normal" character, though of course they may end up a lot weaker than when they were boss characters (expect their HP to be divided by a factor of 10). This may be due to a Redemption Demotion or for PVP balancing. It's also possible that the game has played up a bad guy as such, only for him to become playable near the end of the game. There is no Heel Face Turn involved; just the bad guy doing stuff on his own or just passively riding the plot along.

Beating the game with/as one of these characters usually reveals a special type of end-game victory screen. A fighting game series of sufficient length will almost always resort to this.

Action Adventure

 * Dr. Ivo "Eggman" Robotnik is playable in Sonic Adventure 2.
 * Earlier than that, Amy in Sonic Adventure, and Knuckles in Sonic & Knuckles
 * Tails in the Genesis version of Sonic the Hedgehog 2. In the Master System version, he is the Dude in Distress. Tikal and Chaos were playable in the multiplayer of Sonic Adventure 2, and believe it or not, you can play as Maria Robotnik in 2 levels of Shadow the Hedgehog.
 * Metal Mario appears as a boss in Super Smash Bros.., while anyone can be made metal with the green box item in Super Smash Bros. Melee and Brawl. Similarly, the original game had Giant Donkey Kong and Melee and Brawl can make anyone Giant with the Super Mushroom.
 * Also, Giga Bowser from Melee becomes playable through Bowser's Final Smash in Brawl.
 * Wario, Meta Knight, Pit, Olimar, Squirtle (who doubles as the starting platform in Poké Floats) and King Dedede (who also appeared in the N64 installment as a background character in Kirby's stage) all appeared as trophies in Melee before becoming playable characters, and Charizard was one of the Pokémon summoned in the first two games before it became a playable character itself.
 * Special note goes to Captain Falcon and Ness, who demonstrate a variant of this trope: They were unlockable characters in the first game, then ascended into default characters in Melee before being demoted back to unlockables in Brawl.
 * Shiva was The Dragon to the final boss in Streets of Rage 2, notable for having a moveset remarkably close to that of the playable characters. In the sequel to that game, he became selectable from a Continue after being defeated in an earlier level - however, he lacked one of his special moves when playable, which was available to a later boss version of him.
 * Vergil in Devil May Cry 3: Special Edition.
 * Any and all NPCs, minor characters, major characters, main characters, villains and supporting characters in the LEGO movie games can and will be unlocked some way or another. Even the ones who have no weapons and can't do anything. You can play as anybody. Newer games let you create your own character using parts from unlocked characters.
 * Most Castlevania games after Symphony of the Night let you play as a boss/rival character after you finish the main game. Examples are Richter in Symphony of the Night (and Maria in the Sega Saturn version), Julius in Aria of Sorrow, Julius again with Yoko and in Dawn of Sorrow,  in Portrait of Ruin(Though Richter and Maria are also unlockable), and Albus in Order of Ecclesia.
 * The Bandana Waddle Dee from Kirby Super Star ends up playable in Kirby's Return to Dream Land, over a decade after his original appearance.
 * Oswald the Lucky Rabbit spends Epic Mickey driving the plot and having a Cain and Abel relationship with Mickey Mouse. In Epic Mickey 2: The Power of Two, he's playable alongside Mickey.

Driving Game

 * Many a character from the Mario Kart series games.
 * Minion was the final boss in Twisted Metal, in subsequent games he's usually featured as a midgame boss and tends to be unlockable, and generally is as strong when playable as he is when he's a boss.

Fighting

 * Samurai Shodown added Amakusa to the playable roster in the third game.
 * Darkstalkers made Pyron and Huitzel playable in the second game.
 * Many Guilty Gear sequels include a playable Justice. Other bosses in the series were playable in their own games from their introduction.
 * Baiken initially started as a secret boss in the original Guilty Gear. Since GGX, however, she appears as a playable character from the very beginning.
 * In the Tekken series, the first game had Kazuya as a main character and Heihachi as an unlockable boss. The second game had Kazuya as an unlockable boss and Heihachi as a main character. Then Heihachi recovered his unlockable boss status in the third and Kazuya became a main character again in the fourth, and that's without getting into Jin... yeah, the whole Mishima family is like this.
 * Lee becomes a main character in Tekken 5. After being an unlockable Law clone he was greatly improved in Tekken Tag and is a notable part of the story in Tekken 4.
 * Ditto Shang Tsung from Mortal Kombat to Mortal Kombat II, where Shao Kahn replaced him as the Final Boss.
 * Also Trilogy had most of the different boss and bonus characters playable (though who exactly depended on the version of the game), 4 had Goro in the home version and Deception had both him and Shao Kahn, and Armageddon had Onaga.
 * Armageddon had everyone. Literally every fighter over the course of the entire series was playable in that one (although Khameleon continued to be Nintendo exclusive).
 * Except Human Smoke.
 * The four Shadaloo Grand Masters (Balrog, Vega, Sagat, and M. Bison) from Street Fighter II were exclusively computer-controlled in the original game, but were promoted to playable characters in Champion Edition and onward.
 * Akuma (aka Gouki) was originally the True Final Boss in Super Street Fighter II Turbo, playable only via a cheat code. He became a regular playable character in Street Fighter Alpha 2, with an alternate "Shin Akuma" version as a secret boss (who was only playable in the PS1 and Saturn ports).
 * In Magical Girl Lyrical Nanoha A's Portable: The Battle of Aces allows you to use the Materials once you beat the storylines of the three characters each of them is based upon (e.g. Nanoha, Vita, and Zafira for Material-S). Needless to say, they are the most powerful characters in the game,.
 * Final Fantasy Dissidia made many villains of the series, including the incredibly popular Sephiroth, (though he had already been playable in Ehrgeiz) into playable characters.
 * Dissidia 012 does it again, adding Gilgamesh into the mix. Also notably--in the first Dissidia game, many fans wished that the Final SNK Boss Chaos was playable (he wasn't, not without hacks). But his new and altered form, Feral Chaos, is.
 * Castlevania Judgment has Dracula and Death as unlockable characters.

RPG

 * Joaquim in Castlevania: Lament of Innocence.
 * Magus in Chrono Trigger.
 * Sarevok in Baldur's Gate: Throne of Bhaal.
 * a.k.a. the Secret Companion in Dragon Age: Origins.
 * Dialga and Palkia in Pokémon Diamond and Pearl and Reshiram and Zekrom in the Pokémon Black and White would count too--they have been played up as destructive rival forces, but naturally, you can put both into your possession.
 * Kingdom Hearts has the entire Organization XIII [bosses] as playable characters for the first time in 358/2 Days, along with Goofy, Donald and Riku (AI-controlled support party members) and Mickey (he would be playable but only briefly if you died in certain boss fights).
 * Riku had an entire campaign to himself in Chain of Memories.
 * Some Fire Emblem games let you unlock the Big Bad and several of his cronies (And some unrelated villains) as playable characters fror Trial Maps. That is, those who don't join you during the main game anyway.
 * Of particlar note is the Black Knight from Fire Emblem Tellius. That One Boss in Path of Radiance, one of the coolest characters in the game, now is playable in a few chapters in Radiant Dawn. And he, in fact, kicks even more ass than before, so players can use him to singlehandedly inflict beatdowns of epic proportions on their enemies. Only problem is that he is a Crutch Character to the max who already has the max level, so using him will cost your other characters much-needed EXP, causing many players not to use Blackie at all.
 * Also from the Tellius games, Oliver (the main antagonist for the portion of Path of Radiance that took place in Begnion) turned out to be Not Quite Dead in Radiant Dawn and can be recruited near the end of the game. Statwise he's not very impressive, but his conversations with some of the other characters are among the funniest things in the game.
 * Valkyrie Profile lets you fight Freya, Brahms, and Lezard Valeth in the Bonus Dungeon, and they join you if you defeat them. You can't use them in the regular game, however, since the bonus dungeon is chosen from the main menu instead of being part of the regular game.
 * Bowser, after being an antagonist in the two preceding games, becomes a playable character (and is in fact the driving force behind the plot rather than being a mere joke) in Mario & Luigi: Bowser's Inside Story.
 * Bowser also starts becoming playable in the Paper Mario series from the second game and onwards.
 * Way before either of those, Bowser actually joined Mario's party let Mario join his party in Super Mario RPG.

Simulation Game

 * In The Sims 2, NPCs can be made playable if you have a high enough relationship score and ask them to move in, or marry them. There are also various Game Mods to do so.

Shooter

 * Touhou Project does this all the time. Every one of the Windows games has had at least one boss villain reappear as playable in a later game.
 * It started after the second-game of the PC-98 series.
 * With 2 characters even getting their own games! (to be specific, Aya Shamemaru with what can be most easily described as photography games cmkbined with the bullet hell of the usual series and Cirno, whose game was a bulelt hell with bullet freezing mechanics.)
 * Halo 2 makes the predecessor's main enemy, the Elites, a playable race in multiplayer. While they aren't technically more powerful, the altered hit-boxes has led to arguments on which model is superior and standardization in the MLG circuit.
 * Played completely straight in "Halo: Reach" where the Elites have more shield strength and faster movement speed than their Spartan counterparts.
 * Hibachi is an unlockable fighter in the iPhone port of Dodonpachi Ressurection after beating the game (Good luck with that by the way).

Side Scroller

 * Zero becoming playable as the Mega Man X series went on was pretty much inevitable; first he was an underpowered extra character in X3, but he later became a fully fleshed out alternative to X.
 * The PSP remake Maverick Hunter X allows you to unlock and play as Vile.
 * The other PSP remake, Mega Man Powered Up, does this with all eight Robot Masters. Of course, only six of them were in the original Mega Man.
 * Also in the same game, Roll and Proto Man were also unlockable each with their own stories. Roll was a background character in the first game while Proto Man made his debut as Breakman in Mega Man 3.
 * Inverted in X7 -- X himself is Demoted To Unlockable.
 * After being X, Zero and Axl's navigator from X5 to X7, Alia is promoted to unlockable in Mega Man X8 with moves similar to X. Unlike X however, she cannot use any of X's armor. Layer and Pallette are also unlockable with moves similar to Zero and Axl, though you have to rebuy the weapons for Layer and Pallete cannot metamorph like Axl.
 * The "Meta Knightmare" sub-games in Kirby: Nightmare in Dream Land and Kirby Super Star Ultra (both of which are Enhanced Remakes of earlier games) make Meta Knight an unlockable player character.
 * The Red Arremer that just spent the past few lives terrorizing you in the Ghosts N Goblins series? He's not nearly as nimble when under your control in the Gargoyles Quest subseries, though his fighting game appearances in SVC Chaos and Ultimate Marvel vs. Capcom 3 show greater shades of his abilities.

Sports

 * You can play as Barry DeJay, the announcer from Backyard Basketball, in Backyard Baseball as an unlockable.

Strategy

 * Hachi the shop owner in the Advance Wars series becomes unlockable in Black Hole Rising and Dual Strike. Appropriately, he is one of the game's most powerful COs.
 * Hawke and Lash in Black Hole Rising become playable in the campaign of Dual Strike.
 * Disgaea 2: Cursed Memories does this quite a bit in the PSP Video Game Remake Dark Hero Days, making almost all of the formerly unusable optional boss characters playable, perhaps the most notable of them being Zetta, who's a Game Breaker due to some retained Contractual Boss Immunity, and the absurdly large hit radius on his attacks.