A unique criteria 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.
 "Warning! Challenger Approaching!" screen
In all games, the text "Warning! Challenger Approaching!" is shown prior to fighting the character. All the games have different screens.
The original game had 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. The original game also uses the same music as the 1P-Game's intro. Melee added the phrase "A new foe has appeared!", and as a first, a silhouette of the character the player is about to face is also provided, instead of a full image. 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.
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. If a CPU won the match, then the player is not given the option to 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.
*If the player had previously used the Erase Data > Hidden Characters option in the Options menu, these characters can be unlocked again if All-Star mode is cleared under the established conditions. **The time required to unlock Mewtwo in calculated in "player-hours" (eg. 2 human players gives 2 hours for every hour of game-time, 3 human players give 3, and four players give 4).
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 Super Smash Bros. 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.
** Toon Link, Jigglypuff and Wolf are not part of the main plot of Subspace Emissary; players must beat the main story first then revisit certain stages to find them that way.
 See also