Heidern/Story

Orochi Saga
Eight years prior to The King of Fighters '94 tournament, a 34 year old Heidern was just promoted to his current rank for his exceptional proficiency in martial arts and strategy. The event took place at a top-secret base in Brazil and his wife and daughter were present for the ceremony. Rugal Bernstein, who caught wind of Heidern's high marks, stormed the base when he was away and killed fifty secret service officers on his arrival. When Heidern hurried back from his duties, Rugal deprived him of his right eye and deemed him unfit to be a part of his collection. Instead, he murdered Heidern's wife and child. Disgraced and alone, he developed his own fighting style and mercenary unit, the Ikari Warriors. Since then, he pursued Rugal to avenge his loved ones' deaths.

Sometime before the King of Fighters tournaments, he found Leona as the last survivor of a destroyed village. The girl was amnesiac and Heidern adopted her to care for her. He originally forbid her to partake in any training yet became lenient due to her inclinations on the matter. His refusal to give her a formal rank is his way of protecting her from some of the dangers on the battlefield. Beginning in The King of Fighters '96, he assigns Leona to take his place in the tournaments while he issues orders from the sidelines.

NESTS Saga
He continues to play a major part in one of the series' story arcs, the NESTS Saga, since the Ikari Warriors are trying to destroy the NESTS group. Heidern and his group found Krizalid's corpse and brought it back to their base, however it would soon be taken by Zero not long after. At one point, his own group was infiltrated by one agent of NESTS who stole the identity of one of his allies and attempted to sabotage their operations. Heidern killed him once he saw an opening.

He participates in The King of Fighters 2001 tournament alongside Ralf, Clark and Leona. They advance through the tournament, but is defeated by the Hero Team with Heidern losing to the leader. Heidern reveals that his reason for entering wasn't to win, instead it was to locate the leader of NESTS using the KOF tournament as a distraction. In the team’s ending, Heidern announces to his group that the NESTS case is closed.

Ash Saga
Months before The King of Fighters 2003 tournament, Heidern sent his men to investigate the flying vessel known as Sky-Noah. When Leona experiences the Riot of the Blood after the 2003 tournament, he decides to pull her out of the following one and stay by her side while she recovers. After hearing Ralf and Clark's personal assessment of Leona, he lets his adopted daughter enter the tournament once more. In the end of KOF XI, the Ikari Team managed to secure Magaki's corpse and intended to analyze it, but the cruise they were on got invaded by Shroom and Rimello. The duo took Magaki's corpse, and Shroom also stole Heidern's eye patch and began to wear it as an insult and proof of his ability. Heidern orders the team to investigate whatever role Those from the Past and Rose Bernstein have in the KoF XIII tournament. He also updates Seth, Vanessa, Ramon and Blue Mary on the current situation and tells the agents to disembark on the navel port so they could infiltrate the stadium and find out what Ash and the clan are planning. He puts Seth in charge of the matters on the side.

XIV Arc
Heidern hands the invitation to the Ikari Team and confirms that he's already done a background check on the tournaments host. He informs the team on the recent reports regarding the distortions in spacetime, along with NESTS activity increasing during the same time period. He tells them to infiltrate as usual.

After receiving the information Maxima provided Heidern dubs the entity as Verse, he tells everyone in the room to continue their investigation. 36 hours after the tournaments conclusion, Heidern confirms that there are bodies that were resurrected and has already commenced a 24 hour surveillance. After the meeting he tells Leona that Gaidel's resurrection has not been confirmed, Leona responds that she's not concerned.