Sheilda

The colloquial terms of Sheilda, Shielda, Z-Shiek, or Zelda/Sheik refer to Zelda and Sheik being used in tandem via transforming in the middle of the match in the  series.

The term is usually used either out of convenience or theoretically, as mastery of two characters with differing playstyles simultaneously requires more dedication than is usually considered to be worth the effort. Furthermore, in many of the main series  games, it is rarely if ever used competitively due to Zelda's consistent and extreme history of disadvantages over Sheik, making it more competitively practical to use Sheik almost exclusively. However, in both Super Smash Flash and Super Smash Flash 2, both characters are considered viable, which in turn makes the playstyle as a whole viable.

Sheilda is not currently ranked on the current tier list for either SSF or SSF2. However, both characters are competent in their own right, meaning that the playstyle is still potentially viable. Furthermore, well-placed and periodic transformations allows their abilities to synergize, such as Sheik's combo potential building up damage for Zelda's early KO ability, along with the vastly different paces of their gameplay making maintaining the proper momentum against them difficult.

Match ups
As a bare minimum, due to the nature of the character, Sheilda's matchups are assumed to be the greater between the matchups of Zelda and Sheik against the given character. It is currently unknown how much the aforementioned added benefits would further skew the matchups in their favor.

Tier placement history
Sheilda does not appear in any tier list for the original SSF. For SSF2, however, their first and currently only appearance in a tier list was in the third and final tier list for v0.9b of the demo, where they were recognized as a separate entity for various reasons. One reason was to address concerns of character counterpicking; namely, reports that players would deceptively call Sheik to influence the opponent's matchup strategy only to use Transform and proceed to play as Zelda, or vice versa. Another reason was that the  tier list was a precedent at the time; Zelda/Sheik in Brawl was considered to be marginally better than Sheik alone, ranked at number 24 as opposed to her number 25, with a larger gap between them in tier lists prior to the current one.

For unspecified reasons, Sheilda has not been included on any tier list for SSF2 since v0.9b.