Hiashi was born only seconds before his twin brother Hizashi, making Hiashi the head of the Hyūga clan and Hizashi a member of the branch house, whose only purpose in life would be to protect members of the main house. Years later, when Neji was born, Hizashi became bitter that his son would never be able to reach his full potential like a member of the main house, causing him to be openly resentful of Hiashi and his daughter. Hiashi was also responsible for branding the family's curse seal on Neji's forehead, further straining the relationship between the two brothers. One day during Hinata's training, overwhelmed with resentment and anger, Hizashi gave Hinata a murderous glare, Hiashi sensed the killing intent and activated the curse seal on his younger brother, causing him intense pain. He told Hizashi to leave as he would no longer tolerate incidents like this, telling him to never forget his duties.
During a visit to Konoha by a Head Ninja of Kumogakure, who had come to the village to sign a peace treaty, a young Hinata was kidnapped. Hiashi however saved her, quickly killing her would-be kidnapper, only to discover that it was the Kumogakure ambassador. Kumogakure demanded Hiashi's head in return for the life he had taken, or risk resuming open warfare between the two villages. Konoha, wishing to avoid another conflict, was compelled to comply, but was not willing to fulfil Kumogakure's desire of getting to study the Byakugan. Their father suggested that Hizashi be used as a body double, since his cursed seal would seal off his Byakugan upon his death. Although Hiashi tried to find an alternative to this, Hizashi insisted, saying that he would finally get to choose his own fate (to make up for his wrongdoings against Hiashi), and save his brother's life. Before going to his death, he asked Hiashi to someday tell Neji about why he had made his decision.
Originally, Hiashi had trained Hinata to become the successor of the Hyūga clan. However, Hinata's lack of confidence ultimately led him to decide she was not worthy. He instead turned his attention to his younger daughter, Hanabi.
Hiashi's general personality is very stern and strict, as he assigned both his daughters arduous training, and rarely compliments their performance. Hiashi was willing to use the cursed seal to punish his brother and nephew for showing aggression towards the main house. This stern nature is especially true with Hinata; when she was assigned to Kurenai, he told her that he was unconcerned about the possibility that she would die, saying that the Hyūga clan had no use for a weak successor. However, he truly cares for them as part of the family as seen in his hesitation to allow his brother to die to protect the secrets of the Byakugan or when he asked Kakashi not to destroy the moon, as he thought Hinata and Hanabi were on it. Similarly, he did not view Hinata's talent or potential weak, but her drive to succeed as the successor of the Hyūga clan. In the anime, Hiashi believed she was worthy of being successor, repeatedly trying to motivate her, but ultimately was convinced by his father to focus on the well-being of the entire clan, reluctantly disinheriting Hinata of her status. In the anime, when Hinata began truly pushing herself to improve, Hiashi openly applauded her. Hiashi was also very proud to hear of how hard Hinata fought against Pain.
While a proud man who openly believes the Hyūga is the strongest clan of Konoha, Hiashi is also very honest, as he believed Hizashi should have been the clan's successor after seeing Neji's pure talent and potential far surpassing both his daughters'. Also having great humility, Hiashi began to have doubts of the clan's caste system after the noble sacrifice his brother made. Knowing of Neji's animosity towards the main house for its treatment towards the branch house, soon after Naruto Uzumaki made Neji question his choices in life, as the one who instilled Neji with such anger, Hiashi decided to make amends with his nephew by telling the full story of Hizashi's sacrifice, even going as far as to bow and apologise to his nephew. He also later decides to train Neji in the Hyūga clan's entire fighting style which was not the norm to pass down secret techniques to the branch house — this showed Hiashi to have become much more lenient and not as strictly devoted to traditions thus starting to unify the clan once more.
In later years, Hiashi is shown to become much more light-hearted and cheerful, doting over his grandchildren Boruto and Himawari. He is also apparently close with Naruto as his son-in-law addresses him as "Father".
Being an identical twin, Hiashi is a mirror image of his brother Hizashi with the exception of an unmarked forehead. He has long, black hair (sometimes depicted as brown in the anime), and featureless white eyes like all members of his clan. He normally wears very traditional, loose fitting robes with a long-sleeved, brown haori. As part of the Allied Shinobi Forces, he donned the standard uniform of the Konoha shinobi, along with a black forehead protector (bandanna-style in the anime) of the Alliance.
Years later, Hiashi began showing noticeable signs of ageing: his hair has started greying, especially around his temples. He is also somewhat thinner, and has visible wrinkles especially around the corners of his mouth, around the eyes, and a prominent crease across his forehead.
As a member of the Hyūga clan, Hiashi possesses the Byakugan: a dōjutsu kekkei genkai which grants him an almost 360° field of vision (with the exception of a small blind spot at the upper thoracic vertebrae), x-ray vision and the ability to see the chakra pathway system. He was able to use the dōjutsu's extensive visual range to watch as a far off village was destroyed in an instant by the Ten-Tails' Tailed Beast Ball. In the anime, his prowess was also shown great enough to perceive the thoughts and feelings of those he looks at, as he quickly knew what Boruto was thinking.
Hiashi is a true master of the Hyūga clan's secret taijutsu style; the Gentle Fist. Hiashi's prowess was first glimpsed at when he killed the Head Ninja of Kumogakure with a single strike to the chakra pathway system around his heart, leaving the fight himself unharmed. During Orochimaru's invasion, Hiashi demonstrated tremendous skill with the Eight Trigrams Palms Revolving Heaven technique, able to expand its range great enough to swiftly repel several Oto and Suna ninja simultaneously. When using this technique alongside Neji's, they were able to combine the force of their Eight Trigrams Palms Revolving Heaven technique to deflect the Ten-Tails' tail without giving any ground. He can even use the Eight Trigrams Vacuum Palm to hit targets out of its normal range. He can also use the Eight Trigrams Vacuum Wall Palm with such force using his both hands, that he could buffet one of the Ten-Tails' tails away by himself.
New Era (Plot)
In the anime, as Boruto begins insisting that he has manifested the Byakugan, Naruto decides to bring him to Hiashi for answers. After doting on his grandchildren — much to Boruto's embarrassment — Naruto and Boruto explain the situation. Upon hearing this, Hiashi educates his grandson on the history of the Byakugan and then decides to test Boruto with a sparring match to see whether he has truly awakened the dōjutsu; Hanabi however decides she will do it instead as Hiashi would end up going all out on Boruto.
Boruto does his best, but ultimately the heiress easily defeated her nephew, leading everyone to conclude that Boruto had in indeed not manifested his Byakugan. Afterwards, it was decided that since it was quite late, Naruto and his kids would spend the night in the Hyūga house. After dinner, Hiashi shared tea the traditional way with Naruto and began discussing his kids. Naruto noted that he was happy to see that Boruto's false declarations about the Byakugan were not a mere attempt for attention, believing that Boruto did indeed obtain something special. He also admitted his relief that after all of Boruto's easy accomplishments thanks to his raw talent, it would be good for him to finally learn what it is like to struggle. He also confided to Hiashi that Himawari has seemingly been unable to access her Byakugan since her first usage, which Hiashi insisted to keep him informed on. He also asks Naruto to bring her again if anything else happens to her; Naruto agrees.
- (To the invading Suna and Oto shinobi) "The Hyūga clan is the strongest in Konoha. Remember it well."
- (To Neji) "Hinata is my precious daughter. And you are the precious memento of my late brother."
- (To Hizashi) "There is no such fate! Your son and my daughter are out there fighting to prove it! This isn't about the branch house protecting the main house, it's about two comrades protecting each other!"