Short answers, pulled from the story.
X-Men vs. Street Fighter is a 1996 crossover fighting game developed and published by Capcom. It was the first installment in the Marvel vs. Capcom series and the third Capcom fighting game to feature Marvel Comics characters. The game introduced a tag team format in which each player selects two characters and can switch between them during play.
X-Men vs. Street Fighter first appeared in arcades in September 1996. It was ported to the Sega Saturn in Japan on the 27th of November 1997, and to the PlayStation in Japan on the 26th of February 1998 and in North America on the 11th of June 1998. The original arcade version was also included in the Marvel vs. Capcom Fighting Collection: Arcade Classics, released in September 2024 for Nintendo Switch, PlayStation 4, and Windows.
X-Men vs. Street Fighter features 17 playable characters: eight from Marvel's X-Men franchise (Cyclops, Gambit, Juggernaut, Magneto, Rogue, Sabretooth, Storm, and Wolverine) and nine from Capcom's Street Fighter series (Akuma, Cammy, Charlie Nash, Chun-Li, Dhalsim, Ken Masters, M. Bison, Ryu, and Zangief).
Each player selects two characters before the match and can switch between them at any point during a single-round contest. The dormant character slowly recovers health while off-screen and automatically enters play if the active character is knocked out. Matches end when all characters on one team are defeated, or the player with the most combined health remaining wins if time expires.
The PlayStation lacked the memory to support the tag team combat system as designed, so the off-screen character could only appear during certain special attacks. Animation frames were cut, the match format changed from a single round to two-out-of-three, and frequent slowdown affected gameplay. The port received a 64% average on GameRankings.
X-Men vs. Street Fighter was never officially released on Sega Saturn in North America. Sega declined to manufacture the required 4 MB RAM cartridge domestically, and major retailers announced plans to clear all Saturn inventory by the second quarter of 1998. Some Japanese import copies were sold at certain Electronics Boutique locations.