During her childhood, She was send to Jinpachi Mishima's dojo and became rivals and later married with Heihachi Mishima. She gave Kazuya birth. She was devoted to the Mishima Clan. Until one day, She collapse due to her Devil Gene to turn from lovable human to an evil and sadistic devil as her mission to marry into the Mishimas was to kill the ambitious clan as a result of the Mishima's curse. She saved Akuma to do her bidding to kill both Heihachi and Kazuya if she fails to kill them herself which she failed to do so when Heihachi killed her by crushing her neck. Despite of her death, she currently lives on as a ghost due to her devil gene.
The existence of the character before Tekken 7 was alluded to twice; the first time was in Heihachi's Tekken 2 stage, in which their names ("Heihachi and Kazumi") are written on the floorboard of the temple in the style of Aiaigasa, a romantic expression to show love between couples (the floor carving mark itself was later seen again in the very first trailer of Tekken 7). The second and last time was in the non-canon OVA film Tekken: The Motion Picture, where she was just referred to as "Kazumi" at the time. Her name was also briefly mentioned in Tekken 2's musical score.
Kazumi is described in her introduction gameplay as being a serious and very determined woman who is ready to fight her opponents along with her pet tiger but she is also mysterious, cunning and seductive as seen in her pre-quotes when she calls her ex-husband, Heihachi. Especially when she summons her pet tiger to attack her opponents. Another of her pre-quotes is shown that she was very sad and disappointed implying that she becomes very upset towards her ex-husband for his actions. In one of her win animations, however, she becomes calm and a worrywart and can sense other presences known to her probably her own son, Kazuya who thinks she was dead and she can't accept her fate because of her Devil Gene. Although when she faces her son in a fight, she convinces him to use his powers which is enough to wake up the city despite the latter wanting to gain more power as his own. Another one of her pre-quotes is shown that she formally bows to her opponent and kindly wants to accept her kindest regard. But in one of her win animations where she calmly wonders while walking away to her defeated opponent and becomes impressed with her opponent's moves.
An interesting note is that Kazumi also shares her ex-husband Heihachi's likeness to train their pets to fight their opponents which both had an affinity for animals. In Kazumi's case, she trains her pet tiger which she can summon as she pleases as part of her combo moves, unlike the latter who trains Kuma but not being summoned.
In one of her win animations, she is shown to pet her tiger happily after defeating her opponent. Same goes to her devil form, Devil Kazumi who happily pets her white tiger.
In Tekken 7: Fated Retribution during Akuma's reveal trailer, Kazumi is shown to have an antagonistic side towards her own ex-husband and her own son while she was talking to Akuma which she tells him to kill both of them for their absolute power as Akuma made a promise to "repay her debt" when she was killed. This behavior is also seen in the Story Mode when she becomes hostile towards her husband and sadistically mocks him for marrying her in the first place. This is due to her family's plot to kill him.
It is later revealed that she truly loved her new family, but since the Hachijo Clan have existed to keep over ambitious men like Heihachi monitored and kill them when deemed too dangerous, leaving her emotionally conflicted. As much as she loved her husband and Kazuya, she was prepared to sacrifice them for the good of the world, foreseeing that Kazuya would go down an evil path if she failed her mission. And to that end, she surrendered herself to her Devil form to gain the power to kill Heihachi when he initiated a hostile takeover of the Mishima Household. An act that would end in failure and her death.
In the end, as she is now currently a ghost after her physical body is dead, she herself feels extremely guilty for following her family's order and being the one who caused Heihachi's downfall when he regrettably kills her and inflicted the conflicts on his entire family, as seen in her Japanese website profile.
In Kazumi's promotional art, her appearance resembles that of a white Japanese empress. Kazumi is a very beautiful woman, she has long black hair with a full fringe and is tied in a large Japanese bun and brown eyes. She also has a mole on her bottom right eye.
She wears a white short sleeved traditional kimono dress with a red rope tied on each side of her shoulders and a red ribbon attached to the back of her shirt which is connected to the red rope and a gray diamond designed obi which is tucked into her kimono dress. She has an off-white traditional kimono dress which is tied in a white karate belt on her waist and both sleeves of the kimono dress are hanging loose with flower designs on her sleeves and her skirt of her dress. She also wears a white and red fingerless gauntlets with a golden shell design tied in red ribbons on each side underneath her arm and wrists. The interior of her off-white kimono dress is red. She also wears white tabi which is tied in red ribbons and traditional Japanese geta. She also has a Mishima Zaibatsu insignia at the center of her waist while wearing the outfit.
At the back of her hair which is just above the nape, she wears a kanzashi shaped like a gold phoenix. During her transformation to Devil Kazumi, her kanzashi can be seen and heard falling off after she was struck with a lightning strike while she was down on her knees and being engulfed with a white haze. It's possible that her kanzashi acts as a limiter device to restrain her Devil Gene.