Hikaru Akatsuki, eldest son of Dr. Nozomu Akatsuki and older brother of Akira Akatsuki, possesses a calm, quiet, and mature personality contrasting with his brother's impulsiveness. Academically gifted, he skipped grades to enter university early. He demonstrates proficiency across diverse activities, including sports and video games, earning recognition as exceptionally capable in everyday pursuits. His defining flaw is an intense protective instinct toward Akira; during crises, this focus on his brother's safety causes him to overlook surrounding dangers or objectives, creating a significant vulnerability exploited by adversaries.

During the DISK project demonstration at Raft prison, Hikaru received an incomplete Energy-class biocode after the Bio-Code Installer's destruction. This allowed him to temporarily release heroes like Thor from their DISKs using the D-Smash process. Thor became his primary partner. Their bond deepened over shared experiences of fraternal conflict—Hikaru's fear for Akira mirroring Thor's grief over Loki's villainy. This partnership evolved to allow synchronization, enabling Hikaru to channel energy to extend Thor's time limit and empower him with Build Up Armor.

His protective nature led to significant challenges. When Red Skull threatened Akira, Hikaru absorbed a Dimension Sphere shard in desperation. The Sphere's dark energy corrupted him, placing him under Red Skull's control. He fought Thor and other Avengers until Hawkeye intervened. Thor absorbed Hikaru's corrupted energy blast, purging the dark influence—a process potentially aided by his biocode's self-repair function. Later, Baron Blood seemingly transformed him into a vampire, exploiting his protectiveness to turn him against Akira and Iron Man. Iron Man discovered Hikaru's biocode prevented full transformation, leaving him under psychological influence. Synchronization with Thor again expelled this corruption.

Hikaru expanded his heroic partnerships, wielding DISKs for Nova after the hero's capture and Doctor Strange to rescue Iron Man from the Dark Dimension. He formed a relationship with mutant Noriko Ashida, helping her accept her abilities, though they separated when she joined the X-Men. A pivotal revelation occurred when villain Ronin was unmasked as his brainwashed father. Hikaru reacted angrily toward Iron Man for withholding this information but assisted in freeing Dr. Akatsuki from Dormammu's influence. His restored father developed a complete biocode, rendering the children's temporary summoning obsolete. Due to concerns over their biocodes' unpredictable evolution, Dr. Akatsuki removed them, erasing Hikaru's memories.

News footage of the Avengers' arrest triggered Hikaru's restored memories. He rejoined the final battle against Dormammu and Loki. Recognizing Loki's sabotage of the helicarrier's weapons, Hikaru exposed the trickery, rallying the crew. In the climax against Loki empowered by Dormammu, he used residual biocode energy to grant Thor the Build Up Armor one final time. He participated in delivering the Kree Mact weapon to Akira for Iron Man's use, expressing confidence in his brother before their ultimate victory.