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)