Short answers, pulled from the story.
Marvel vs. Capcom: Clash of Super Heroes is a 1998 crossover fighting game developed and published by Capcom for the CP System II arcade system. It was exhibited at the AOU Show in February 1998 and released in Japanese arcades that same month, followed by North American arcades in March 1998.
The game features 15 regular playable fighters plus six secret characters accessible by inputting codes on the character select screen. It also includes 22 Special Partner characters that can be called in for support during battle.
The Variable Cross, also called the Duo Team Attack, is a technique that allows both characters on a team to fight simultaneously for a limited time. During a Variable Cross, the player gains unlimited use of the Hyper Combo Gauge, enabling multiple heavy special attacks in quick succession.
The original PlayStation's limited RAM could not support the four-character tag team system from the arcade version. Capcom removed tag team battles from most modes to preserve the game's speed and graphical integrity, reducing the secondary character to an assist role instead.
Marvel vs. Capcom Origins, the 2012 HD compilation that included Clash of Super Heroes, was removed from online stores in December 2014 following the apparent expiration of Capcom's licensing contracts with Marvel Comics in 2013.
In Japan, Game Machine ranked it the second most successful arcade game of March 1998. In the United States, the game sold an estimated 3,000 arcade units and reportedly outperformed Street Fighter III to become Capcom's most successful arcade game in the US during that period.