An Alpha Counter (known as a Zero Counter in Japanese) is a defensive counterattack technique first introduced in Street Fighter Alpha. Variations of the technique also appear in The King of Fighters, where they are known as Guard Cancel Blowbacks, and in Marvel vs. Capcom 2 where they are known as Variable Counters.
Alpha Counters in Street Fighter Alpha
In Street Fighter Alpha, when executed by performing or while blocking an attack, the character will perform a quick counterattack, usually one of their special moves, at the expense of 1 level of super meter. In the original Street Fighter Alpha, pressing or was character dependent. Beginning with Street Fighter Alpha 2, each character has two different Alpha counters; one performed with and the other performed by inputting .
Guard Cancel Blowbacks in The King of Fighters
In The King of Fighters, a character can perform their standard Blowback attack by pressing + (+ in M.U.G.E.N). Blowback attacks can also be done while blocking an attack under the same conditions as Alpha Counters can be done in Street Fighter Alpha. The Guard Cancel version of the Blowback attack, however, has a much faster startup than the normal version and inflicts much less damage.
Variable Counters in Marvel vs. Capcom 2
In Marvel vs. Capcom 2, they are instead executed with or . Instead of counterattacking using one of character's special moves, however, it will instead call in one of your two assist characters to perform the counterattack. They are otherwise functionally identical to their Street Fighter Alpha counterpart.