An unlockable character (also referred to as a secret character or hidden character, and in-game as challengers) is one that is not available at the start of the game (unlike a starter character), but instead must be received in-game (unlike downloadable characters) through other means.
A unique criterion must be met in order to unlock a certain character, as outlined below. Once the player has met this criteria in-game, a message will display; in all cases, the player is given this screen after completely finishing gameplay.
"Challenger Approaching!" screen
The "Challenger Approaching!" (挑戦者が現れました！, A Challenger has Appeared!) screen is a message that will be displayed after the player has met a criteria required to unlock any of the available characters. Every game has its unique screen.
NTSC versions of the game had a silhouette of the character merely displayed in a blue box in front of a black background, with an exclamation point in a red circle on the top of the screen, and the phrase "Challenger Approaching" next to it. PAL and Japanese versions had the approaching challenger fully revealed. The original game also uses similar music to the 1P-Game's intro, with the only difference being the absence of drums for the "challenger approaching" tune. Melee added the phrase "A new foe has appeared!," and also had a silhouette of the approaching challenger. The poses for the challengers were silhouettes of their renders when selected on the Character Select screen (some, however, like Ganondorf's and Luigi's, were their renders from inside the boxes). The game also added animations to the screens, with an undulating matrix effect in the background, while the exclamation point, text and silhouette would be eased in. Original music was also provided solely for the screen, which took the sound of a slow, ominous siren. Brawl returned to the plain black background (there would be a silhouette of an unlockable character the player would face), but still had animations that would ease in, and when the player pressed any button, the images would fade to black, while the "Challenger Approaching!" text would scatter all over the screen; the siren is also considerably faster and more panicky in its sound than how it sounds in Melee. Smash 4 reused Brawl's warning siren, but added in considerably more colours, including hues of green, black, and violet; the screen also says "A challenger approaches!" instead of its previous quotations. However, the Wii U version kept the previous quotations.
After the "Challenger Approaching!" screen, the unlockable character will then be fought in an ordinary one stock match with no time limit, on a pre-determined stage and pre-determined music. The player uses the last character played as, as well as the same palette swap; in the case of multiplayer battles, the player who won the match will fight the challenger for the first three games, though if a CPU won the match, then the players are not given the opportunity to unlock the character. Super Smash Bros. 4 changed this formula, and it is no longer necessary to win a multiplayer battle in order to fight the challenger; if a CPU wins the match, then the human-controlled player with the highest rank will fight.
By defeating the opponent, the character becomes playable. If the player fails, however, they will not unlock the character, but in the next battle they win, they are guaranteed to meet with the challenger again.
The "Challenger Approaching!" screen has become somewhat of a meme in the Smash community. Many digitally manipulated versions of these screens have appeared, often featuring characters who do not appear in the games, mostly using artwork of the character from another game instead of original artwork (which gives away its falseness). The screens became so popular that they were featured as part of the DOJO!!; in earlier updates, whenever a new character was introduced, a banner that said "Warning! Challenger Approaching!" would appear above the entry.
Masahiro Sakurai once stated that he feels that it would be appropriate that third-party, or "guest" characters, like Solid Snake or Sonic the Hedgehog, would be unlockable. This became true when Brawl was first released in Japan.
In addition to the challenges and versus match method of unlocking, all unlockable characters can be unlocked by playing through The Subspace Emissary. The majority of these characters are unlocked as part of the linear story of SSE. However, Jigglypuff, Wolf, and Toon Link are only unlockable after the main SSE gameplay is completed by finding a secret area while replaying levels. Once a character joins a group of playable characters, the character will be instantly unlocked for normal play (without requiring a "Challenger Approaching" battle). However, some characters, as per the SSE's story, must be battled before they will join the player's party, such as Bowser.
Sonic is the only character who can be unlocked 4 different ways.
In addition to the characters below, Mii Fighters are not shown on the character select screen until the player has created at least one. However, since the ability to create Mii Fighters is granted from the beginning, the character cannot truly be called unlockable.
Unlike Super Smash Bros. for Nintendo 3DS, Bowser Jr., Jigglypuff, Ganondorf, and Ness are starter characters. Like the 3DS version, a character is unlocked for every 10 versus matches played. Two exceptions to this are Mr. Game & Watch and Duck Hunt, which they need 20 more versus matches than R.O.B. and Mr. Game & Watch, respectively.
In addition to the characters below, Mii Fighters are not shown on the character select screen until the player has created at least one. However, since the ability to create Mii Fighters is granted from the beginning and there is no Challenger Approaching fight, the character cannot truly be called unlockable.