Alright well to start off, I'm going to give just the short style background of Hiyori that way she's a tad more relavent than "Oh its Hinata's mom".....
Personality and History
So to start, Hiyori Hyuga obviously has a gentle and pacifistic personality. Like her daughter she hates fighting. Due to this pacifistic personality. When placed in a combat situation, Hiyori resolves to end fights as quickly as possible. Hiyori has an uncanny ability to understand people's emotions and feelings...allowing her to better read what they intend to do in combat, or to simply build better bonds of empathy/sympathy. She also possesses an above average intellect (not a genius level, but not normal). Ironically however, she can be pretty dense at times. Due to her being brought up properly as a member of the Hyuga clan, she tends to be very punctial (another trait she shares with Hinata). However, unlike her daughter, she tends to be completely oblivious to things such as Sacrcasm leading her to become completely confused at times...as she will typically take someone's sarcastic comment as though there was no sarcasm (no matter how obvious it was). She also tends to forget the idea of feminism when it comes to combat....typically if she does have to go into battle, she'll strip off the top of her shirt leaving nothing but some wrapping to cover herself (like Erza Scarlet if you've seen Fairy Tail). While those like Kushina typically react in hostility (partly due to Hiyori seemingly degrading herslef....and a little bit of jealousy ), Hiyori only thinks of it as the practical move to do because it provides her with greater flexibility and manuverability in combat.
In battle, her pacifistic tendencies usually lead to her to try an coax a peaceful solution; however, if it is 100% unavoidable, her aim is to kill the opponent mercifully meaning that it is as quick and painless as possible (for example....a blow to the temple). However, due to her extreme desire to make the deaths painless...often times it ends up making her appear more brutal and heartless than actually gentle due to her emotionless facial expression. However, this is actually a reflection of her conflicting inner morals....her struggle between having to end the fight to protect her friends and her overal pacifistic nature. Due to her more brutal seeming nature in combat, she earned herself the nickname "The Serenade of Madness," by the few who managed to surrender before it was too late, and, "Mentally Insane," by Kushina.
As a Genin, Hiyori was a part of Team 8 alongside Kushina Uzumaki and Mikkoto Uchiha. Similarly to her daughter, Hiyori generally followed behind the orders of Kushina and Mikkoto who would tend to fight each other as rivals for leadership. However, during missions or situations that called for deciscive actions, Hiyori would take command...usually asking kindly....and due to her frightening combat skills...it usually was responded to by a fearful submission by the other ladies of Team 8.
Aside from being close to Kushina and Mikkoto, Hiyori was very close friends with, surprise surprise, Minato Namikaze. Our future Yondaime Hokage found that amongst his class, his greatest rival was that of Hiyori Hyuga, and as a result he made her his rival. Unfortunately for him...it always ended in his defeat. While he possessed a superior combat skill and jutsu arsenal (FTG...duh ), his fault lied withing Hiyori's Teikokku Byakugan as well as her ability to understand those around her.
Due to her peaceful and understanding nature, Hiyori understood the thoughts that could go through people's heads...adding the insight of her Byakugan, she was practically peerless against those whom with she was close friends with. Defeating them because she knows what actions they will take as opposed to being a supereior combatant.
As we know, Hiyori was married to Hiashi Hyuga....despite her dislike of fighting and her gentle nature, she ended up falling for Hiashi for his sense of order and his calmness (as they brought a breeding ground for peace, serenity, and gentleness), and a slightly kinder personality he typically hides from his Image as the Hyuga head.
While it seems a tad incest like at the start...the Hyuga Clan is notably a large clan...large enough to have huge numbers of branching clans...some of which can form almost from outside families marrying Hyuga members. So...no incest !
Finally, Hiyori Hyuga was a leader of a Genin Team for a short period of time. Her most notable student: Might Gai. With her kind and encouraging nature, Hiyori encouraged Gai to always to his best....and she was easily one of his inspirations during his career. During an early morning sparing session with his teammates, Hiyori noted Gai's youthful energy.....the compliment from his idol sensei would inspire Gai for the rest of his career. Additionally, after an observation of Hiyori and Minato's daily sparing sessions...he became determined to find an eternal rival of his own....naturally picking from his sensei's rival's team...Kakashi Hatakke.
Combat Style
Hiyori Hyuga is one of the few Hyuga who use the Teikokku Byakugan. As such, she has a full 360 degree vision, control and sight of Natural Energy, can see the elements people build up, and finally has mastery over her own elemental affinity which is either Fire or Lightning (can never decide which one to give her...for now I'll stick with Lightning).
Hiyori Deploys her control over Natural Energy and Raiton styles similarly to the Raikage meaning that she uses it to boost her speeds to above human levels (aquiring Gates Level Speeds if not greater). It is due to this ability, she was able to react to Minato's speed. Adding her understanding of him....this is the explaination to how he always lost against her.
Notably like her daughter, Hiyori is not practical with the normal Gentle Fist style because those who are peaceful...do not do so well with its rather Grim and painful practices....as a result, she lacks complete mastery or profiecinecy in Jyuuken, but it is made up through the application of her Raiton Element.
One of the Key features of those who use the Teikokku Byakugan is the Twin Lion Fists. This manifistation of Shape Manipulation is a technque that in its base form is similar to any Teikokku user (Namely...a Pair of Lion Heads). However, upon activating her eyes, Hiyori comes to evolve them into SoTiKen (Twin Tiger Fists)....her Fists evolving to turn yellow with crackles of lightning, and take the shape of a Sabertooth Tiger. Also Notable is that the "Eyes" of the technique take the same spiral shape as the Teikokku Byakugan.
In short, when in combat she cannot avoid, Hiyori will attempt to use her speed in order to obtain a clean one hit painless kill from her opponents...and uses her understanding of people to best predict their actions. It is a flaw in her combat style as well because it can force her to go out of her way or take too much chakra in an attempt to do so (if fighting say a high level shinobi that can match her speed).
Role in the Story
During the waining days of Kushina's pregnancy, Hiyori grew increasingly concerned for the safety of her former teammate and close friend...due to the enemies the Yondaime had built as well as the Kyubi within Kushina. While wishing to have been of help, Hiyori was unable to provide any assistance because she herself was busy holding a child of her own....her second child that would be Hinata (makes more sense once I introduce Tsukasa). In an effort to calm her concerns, Minato decided to promise her that once his child and her soon to be born child grew up a little, they'd all go out for a picnic to make the two great friends (naturally not knowing the genders of their children...or the epicness of NaruHina to come ).
Despite her intelligence, Hiyori was dumbfounded, but not in the condition to point out the flaws (as in her mind she naturally assumed this would be a given occurance) so she merely said that he had better uphold his promise.
Naturally she was grived to watch the terror of that night and the loss of two close friends...She would then be a loving mother up until the five years before the start of the story.
Tensions had been high for quite some time between Konoha and members of the Uchiha....Hiyori could easily see the personality actions and shifts in Mikkoto Uchiha. Though...she was unable to do anything. Finally, the dreadded Massacre night occured. Looking out at the moon, she shed a tear as she watched Itachi moving through the village towards his home. A second tear as she watched her son move as well.
Then what she did not expect to see....her little Hinata walking out.....trying to sneak up on her loving brother. At this, Hiyori had to take action. Moving through the village and coming up to her daughter just moments before a shadowy figure with long hair and a mask appeard in front of her daughter.
Under the premise of keeping no witnesses, Obito was prepared to end the life of the Hyuga child, but he quickly found himself between two Hyuga of great talent. Unable to take any action as the result would prevent a completed massacre, he struck a deal with multiple conditions.
1. Hiyori Hyuga would donate some of her DNA to the man.
2. Tsukasa Hyuga was to leave Konoha and become a rogue shinobi.
3. In the following month, Hiyori Hyuga must die.
4. The memory of Tsukasa Hyuga must be struck from the Hyuga clan.
In return to taking out two potential thorns to his plans, Obito promised to grant the life of Hinata Hyuga.
Without any option to argue, Hiyori instantly agreed....as did her son. What happened with Tsukasa I'll detail you later.
Between Sharingan Genjutsu, and the Hyuga Kinjutsu Heaven's Concealment(genjutsu...whoohoo!) the memory of her sibling and the events of tonight were sealed away inside Hinata's subconcious.
Then, using the Heaven's Containment (simply an adaptation of Heaven's Concealment, but using the Curse Seal Mark as a medium) the Side Branches were forcefully erased of Tsuakas's existence. The Main Branch swearing an Oath of Silence to never speak of him.
All that remained was Hiyori's death. Through her time as Minato's friend...and countless hour helping Minato shift through the remnants of the Nidaime Hokage's techniques...she concluded the possibility that she could be revived with the Edo Tensei. As a result, she knew she had to spend her final month making the preparations of ensuring her defeat were she to be summoned to the battlefield.
For the last month of her life, Hiyori would spend them training her daughter in the arts of Hyuga War Sages (those who fought with the Teikokku Byakugan). While normal circumstances of learning the War Sage Style is the culmination of a peaceful and slow process....Hiyori had no time, and as such had to awaken them through more brutal means....combat.
Until Hinata could awaken her Teikokku Byakugan...she had to spend every waking moment fighting her mother. Her only breaks would be when she was knocked unconcious, or for three meals scheduled in the day. Needless to say, this was a living nightmare for Hinata. She had to fight the person she loved most in her life, and she could never give up until she could strike at her with lethel power.
After being pushed far beyond her physical and mental limits....the pent up grief, anger with herself, and desire to stop fighting in order to return to a peaceful life...Hinata's Gentle Furiocity exploded out.
Hiyori's origional intention had been to awaken these eyes, bind them, and then proceed to end her own life (in order to complete the contract)...she was unable to contemplate the reactions pressing her daughter into a corner would bring. The resulting awakening of Hinata's eyes, exponentially and upredictably raised the power of Hinata's Lion Fists......and was caught off guard as her daughter landed a blow to her stomach that sent her rolling across the hill top in which they trained.
Hiyori's task had finally been complete, immedietly recovering, she swiftly used yet another Hyuga Kinjutsu: Heaven's Requiem. The Fuinjutsu developed through ancient ties with the Uzumaki clan established a contractual seal.
To put it simply, Hintata's eyes would be sealed...causing her to be unable to use the War Style or her Teikokku Byakugan unless certain requirements were met. The contract formed was that her eyes would not awaken unless Hiyori appeared before her daughter as an enemy.
With her final actions taken, Hiyori collapsed to her knees, and with pride and happyness embraced the daughter she no longer had to face....but she knew this was her end. The blow that Hinata had landed had miraculously managed to pass through the minor Raiton shield she had established....and now the internal organs that Hinata had blasted into with her Jyuuken strike were immedietly beginning to fail.
Hinata could only stand traumitized while crying apolgies of forgiveness as her mother slowly and painfully faded into the next life. Though her daughter couldn't make it out through her cries....Hiyori departed her life thanking Hinata for being her gentle child. Through her final acts...Hiyori died a mother. Saving the life of her precious daughter, and providing her with a chance to live her life in peace...though it would turn out that Hinata would strive to be a Shinobi that was Kind like father, and gentle like her mother.
