Gau Ban's origins differ slightly between the OAV and anime.   In the anime, bandits attacked him home village when he was nine years old, slaughtering everyone, including Gau's parents. Over the next year, Gau exercised guerrilla-style attacks against the bandits for revenge. He eventually encounters Elle on the road by chance.   In the OAV, Gau is ten when the attack occurs. Seeing his parents murdered in front of him, Gau snaps and attacks the bandits. He kills most of them, but then his luck runs out. Elle shows up before he himself is killed, and finishes off the remaining bandits.   Since Gau is the only survivor of his village, Elle asked him, "Do you want to be strong?" and takes him with her when he answers yes. Originally, Gau only intended to follow Elle in order to steal all her money and discover the source of her powers. After some time, however, Gau came to care for Elle like an older sister.   Over the years, Gau has trained side by side with Elle in an effort to become strong. With his dedication and signs of promise, he quickly catches the attention of the 57th Sevar, Scarface, who decides to help Gau improve, albeit only indirectly. Many incidents that Gau encounters are in some way or another engineered by Scarface, for the purpose of driving him to improve and become stronger.   He also catches the love-interest of Kyuo Lyu, however Gau is completely oblivious to her intentions. In an ironic turn of events, Scrib Lowengren, King Iba Sutra's apprentice in Open Skills and effectively Gau's counterpart in the race to become the next Sevar, falls in love with Kyuo at first sight. A love triangle instantly develops, and so both Lowe and Gau become rivals in more ways than one.   The two of them battle each other several times, Gau eventually coming out the overall victor. In respect, Lowe gives Gau his earrings, which Gau wears during his most important battles.   During Solfan's initial invasion into Ashlianna, Gau is assigned by Scarface to protect Diaz Ragu, Gau's adopted older brother via Elle, and who Gau already considers to be a older brother figure to him. While this decision is tactically sound, Scarface is also testing Gau so that he will become stronger; Solfan itself desires to use the town of Blorahan, where Diaz lives, as a staging point, as well as Diaz' skills as a weapons crafter.   During what later becomes known as the Lightning Battle, Gau is given the Black Wing boomerang by Diaz to use, and so creates the nickname for himself, "Black Howling". During the other fights that come, Gau successfully defeats every opponent that came his way, including G, the insane former 58th Sevar who had betrayed Kuruda and later sides with the Solfan forces.   When he is forced to fight the non-human being Darkness, another test by Scarface, Gau is led very nearly to death, his neck slashed open. The test fails, however, as Gau is paralyzed by his fear of death, and Diaz Ragu intervenes on his behalf. Gau eventually defeats Darkness in the end, mastering the Shadow Gate Genesis technique Cannon in the process, but Diaz dies moments later, his 's failing health body finally succumbing to his illness. Wracked with grief and self-loathing over this, Gau slides into a deep depression, eventually coming out of it only with Elle's help.   During Solfan and Ren Fuuma's assault on Kuruda, Gau trieds to fight him but is quickly overwhelmed by Ren's ferocity. During this time, imminently faced with death, Gau finally realizes what Darkness had been trying to teach him during their battle: that man's true strength comes from the purity of a heart unburdened by doubts, like a beast's. Armed with this newfound understanding, Gau overcomes Ren's attacks with ease, and defeats him.   Recently in the manga, it has been revealed that Scarface is Gau's older biological brother. Their father's name is Shere Kahn, the founder of Kuruda-style Kōsappō Yin-ryū, Ren Fuuma's superior, and the one who masterminded the attempted destruction of Ashlianna. Who Gau had thought were his parent were merely the people he had been entrusted to.   (Source: Wikipedia) 

In the land of Kuruda, warriors with magical powers and incredible fighting skills battle for the ultimate prize: the title of Sevaar, the strongest warrior in the land. Elle Ragu, nicknamed Shadow Skill, is the newest Sevaar to emerge, but that doesn't make her life any easier. Teaching her "little brother," Gau, how to be a warrior, fending off assassins from other kingdoms and thwarting enemy invasions is hard enough, but her biggest challenge will be paying off her drinking debts.
 
(Source: Anime News Network)

The story takes place in warrior kingdom where a deadly martial art called Shadow Skill is practiced to defend the continued existence of the kingdom. Dreaming of becoming an invincible warrior one day, like the legendary warrior named Scar Face who has defeated 2,000 opponents, the boy named Gau Ban trains hard with his older adopted sister Elle who has mastered Shadow Skill. A deadly combat breaks out when a rogue pack of intruders suddenly assaults the castle to steal a box that contains a secret...
 
(Source: Anime News Network)

In this OVA, the story of Gau's history with his adopted sister and the secrets that make up the shadow skill unfold. The pair make their annual return to the gravesite of Gau's parents, where his growth and progress as a warrior can be exhibited for them. Gau has come along way in his training as a Sevalle under Elle's mentorship, but there are those who wish to find a way of eliminating them, just waiting for the right opportunity to mount a deadly attack. As the travellers make their journey, their family bonds and skill will be tested. Opponents are many, and battles are fierce, but Gau's training also entails taking on his sister in battle, and fully embracing the skills he must learn to defeat his ultimate opponent. 

Gau Ban is a boy who is struggling to become a "Sevalle," the strongest class of warrior in the mercenary kingdom of Guruda. Gau's elder sister, Elle Rag, is the 59th "Sevalle" and holds the honorable title of "Shadow Skill." The story develops around the two siblings, as they fight magical monsters and historical warriors. The major part of it is a battle called "Shadow Skills" that is fought with only the hands and feet. It begins when the historical warriors chant the words "We are invincible!" and obtain superhuman powers.


In a war torn world of deadly conflict, one small group of heroes stands against the forces of darkness, armed only with two weapons&mdash;their unswerving dedication to good and the superhuman martial art known as the Shadow Skill...
 
(Source: Anime News Network)