Saisyu Kusanagi is a character who debuted in The King of Fighters '94 as a victim in Rugal's gallery of defeated fighters. He appears as a mid-boss in The King of Fighters '95 and becomes a regular character for the first time in The King of Fighters '98. He was originally meant to have died in KOF '94, but was added in at the last minute of developing stages for KOF '95.
Introduced in The King of Fighters '94, Saisyu Kusanagi (Kyo Kusanagi's father) is the current patriarch of the Kusanagi bloodline and master of the sacred flame. Unlike his son, he dons an old fashioned style Kusanagi clan outfit and utilizes more basic traditional martial arts techniques. In 1994, Saisyu embarked on a quest to challenge fighters from around the world. Kyo and his teammates (Benimaru Nikaido and Goro Daimon) learn of his demise just before the final match in KOF '94. Saisyu apparently challenged Rugal Bernstein and is beaten badly, it is implied that he allegedly later succumbed to the injuries he sustained before the finals.
However in The King of Fighters '95 it is eventually revealed that Kusanagi survived, was then brainwashed by Rugal's secretary Vice in order to compete as a mid-boss in the semi-final round. He is supposedly defeated by his son Kyo and his teammates. Upon his second defeat, Saisyu goes in to self-exile, only to resurface again in (a non-canon event) The King of Fighters '98. In The King of Fighters 2001, he appears before a distraught Shingo Yabuki (Kyo's student) and tutors him in the traditional methods of the Kusanagi Martial-Arts style, as seen in the Japan Team's ending.
By the time Shingo completed his training in The King of Fighters XIV, Saisyu senses an evil presence disrupting time and space. He advises his son to meet up with Tung Fu Rue and investigate the matter during the tournament.
Saisyu Kusanagi is simply an older, more mellow version of his son Kyo. Regardless, he is just as arrogant and cocky as Kyo; like father, like son. This is quite evident based upon his attitude and body language during several win-poses; where he either laughs abrputly after winning a match (i.e. not taking the bout seriously), nonchalantly scratches his beard or generally seems uninvolved... or bored by rubbing his abdominal muscles in a laxidasical show of bravado.
The only characters that seem to make him even remotely contrary are Rugal (who defeated him back in KOF '94) and seemingly to a lesser extent Kyo (his savior, who subsquently also defeated him a year later in KOF '95). Ironically enough he seems to harbor no indication of resentment or ill-will in the slightest towards Vice (one of Rugal's secretaries; a dominatrix who indulges in sadism), she brainwashed him in KOF '95. Vice visibly holds Saisyu with contempt during their pre-fight scene, by pointing at him and spitting. However he assumes his fighting stance and never acknowledges her insulting behavior.