The King of Fighters '99: Millennium Battle is a 1999 head-to-head fighting game by SNK released for the Neo Geo arcade and home platform. It is the sixth installment in the company's The King of Fighters series. The game was ported to the Neo-Geo CD and the PlayStation as the final KOF game released for both platforms. A Dreamcast version was also released under the title The King of Fighters '99 Evolution. Both the Neo Geo and Dreamcast versions are included in The Kings of Fighters NESTS Hen compilation released for the PlayStation 2 in Japan. The game introduces several changes to the established KOF format, as well as a new story arc that would later be dubbed the "NESTS Chronicles".
Instead of the three character teams from previous KOF games, each team now has four members. Before a match, the player chooses three of the characters to use in the actual fights, while the fourth member becomes the designated "Striker", a character the player summons during battle to help their character out by performing one of their Special Moves against the opponent. A Striker can only be summoned a limited number of times during a single match, which is determined by the number of "Strike Bombs" at the bottom of the screen.
The selectable fighting styles (Advance and Extra) from KOF '98 are gone. Instead, the game has a single playing style modeled after the Advance mode from the previous game, where the player fills their power gauge by attacking the opponent or performing special moves. This time, there are two powered up states the player can choose during battle depending on the button combination used). Counter Mode increases the player's offensive strength and allows the player to use their character's Super Special Move infinitely, as well as combo a Special Moves into a Super Special Moves by using a "Super Cancellation Attack" or a "Moving Attack". The other powered up mode is Armor Mode, which increases the character's defensive strength, allowing the character to take more damage from the opponent. However the player cannot use Super Special Moves during Armor Mode.
Source: Wikipedia, "The King of Fighters '99", available under the CC-BY-SA License.