Short answers, pulled from the story.
Street Fighter Alpha 2 is a 1996 arcade fighting game developed and published by Capcom, the sequel to Street Fighter Alpha: Warriors' Dreams. It first appeared publicly at the February 1996 AOU show in Tokyo and was ported to the PlayStation, Sega Saturn, and Super Nintendo that same year. Street Fighter Alpha 3 followed in 1998.
The Custom Combo system, called the Original Combo in Japan, replaced the Chain Combos from the original Street Fighter Alpha. Players activated it by pressing a combination of punch and kick buttons while the Super Combo gauge was at level one or above, then chained any sequence of moves until a Timer Gauge at the bottom of the screen ran out. Capcom senior planner Shinji Mikami designed the system to make normal hits more central to gameplay than special moves.
Street Fighter Alpha 2 added Dhalsim and Zangief from Street Fighter II, Gen from the original Street Fighter, Rolento from Final Fight, and Sakura Kasugano as a new character who took up street fighting after witnessing Ryu in battle. M. Bison, Akuma, and Dan became immediately selectable, and Shin Akuma was introduced as a secret opponent more powerful than the standard Akuma.
Gamest magazine voted Street Fighter Zero 2 the Best Game of 1996 at its Tenth Annual Grand Prize, with Dan Hibiki and Sakura Kasugano on the cover of that issue. The Sega Saturn port sold over 400,000 units in Japan. Electronic Gaming Monthly later named the Saturn version the 36th best console video game of all time, chosen specifically for its superior graphics.
Street Fighter Alpha 2 is the title used in North America and Europe; Street Fighter Zero 2 is the Japanese and Asian release of the same base game. The Western arcade version added three characters selected by Capcom USA: Evil Ryu and EX versions of Dhalsim and Zangief. A separate Japanese update, Street Fighter Zero 2 Alpha, later added EX versions of Ryu, Ken, Chun-Li, Sagat, and M. Bison based on their Street Fighter II: Champion Edition designs.
The Sega Saturn version is the most highly regarded console port of Street Fighter Alpha 2. Electronic Gaming Monthly named it the 36th best console video game of all time, chosen specifically for its superior graphics over the PlayStation version, and cited exclusive frames of animation and a Saturn-only illustrations mode. The Saturn release also uniquely offered Evil Ryu, EX Dhalsim, EX Zangief, a survival mode, and an art gallery absent from the PlayStation port.