— Arai in Yakuza 4
"You can't turn back the clock. Regrets serve no purpose."
Arai appears to be a man of integrity and noble intentions at first. He is shown to share mutual respect with his subordinates in Kanemura Enterprises, and with Akiyama. He was willing to help a then-homeless Akiyama retrieve money that was stolen from him by thugs. However, deep down, Arai is highly ambitious and manipulative; he pretends to be loyal to the Tojo Clan but is secretly superior to Munakata who he also later betrays. He is willing to sacrifice and betray those around him in pursuit of his plans, including Kazuo Shibata whom he kills, Munakata who he attempts to kill, and even his own oath brother Takeshi Kido whom he only spares in the interest of determining his allegiances. Despite being a police infiltrator, Arai seems to have some degree of respect for the yakuza and desires to be one; when he attempts to kill Munakata, he states that he lives by his own code, not just as an officer of the law, but also as a yakuza. He also, in yakuza tradition, chose to strip when fighting Akiyama.
Arai wears a black suit, the coat he wears is buttoned up halfway. He wears a midnight blue dress shirt with his tie being a lighter color. There is a little badge attached to the left flap of his coat. He wears black dress pants, and wears black leather dress shoes.
Arai was originally a police infiltrator sent to infiltrate the yakuza.
He first appears in a flashback that takes place during the climax of the first game. When money rains from the sky following the explosion at the Millennium Tower, he helps a then-homeless Akiyama get his money back from the thugs who try to steal it. This creates a friendship between the two of them, and leads to Akiyama and his company forming a mutual relationship with Arai's yakuza group.
At some point, he became sworn brothers with a junior member of Kanemura Enterprises, Takeshi Kido. Arai is shown to be remarkably competent despite him being merely the lieutenant of a third-tier Tojo Clan group, with Kido speculating that he might even be able to lead the Tojo Clan one day. Arai had also formed an allegiance with Isao Katsuragi, through Shibata co-conspiring with him.
Arai instigates the conflict of the game when he murders Masaru Ihara at the Elnard club. In reality, the altercation at Elnard and Arai's shooting of him was part of a plot by Katsuragi and Shibata to coax the Tojo into giving Katsuragi and Ueno Seiwa the lucrative Kamurocho Hills project. Katsuragi demanded of the Tojo's chairman, Daigo Dojima, that he either hand over Arai's head or forfeit the project, but Arai had secretly been in hiding under his and Katsuragi's knowledge the whole time.
Arai later appears in Part 3, when Masayoshi Tanimura closes in on him and Shibata. Shibata wanted to rape and murder Yasuko Saejima in retaliation for her murdering many of Shibata's men on Katsuragi's orders. At this point, Arai revealed his loyalty to Katsuragi, and killed Shibata, stating that this was in retaliation for Shibata blackmailing Katsuragi and due to Shibata no longer being necessary to his plans. He then escaped from Tanimura.
He appears again in Part 4. Herein, it is revealed that Kido is also conspiring with Arai, though in reality Kido is secretly loyal to Daigo. After Kido shoots Katsuragi (unaware that he was wearing protection) and hands over Munakata's file provided by Kazuma Kiryu to Arai, Arai shoots Kido. Kiryu scolds him for betraying an oath brother, declaring that he will never lead the Tojo Clan. He leaves without taking Akiyama's money.
Arai then reports to Munakata. Here, he is ordered to kidnap the children of Kiryu at Sunflower Orphanage, as a bargaining chip to force Kiryu to return the money. At this point, Arai states that he lives by his own code as a yakuza and an officer, and shoots Munakata.
He finally appears at the rooftop in the finale, lured there thanks to the money. Here, he confronts Daigo, reprimanding him for betraying the Tojo Clan and sending Goro Majima to prison, and reveals him being a police infiltrator. It is revealed here that Arai intentionally ensured Kido would survive his shooting; he was suspicious that Kido was working on Daigo's orders, and when Kido ran to Daigo after surviving, this verified his suspicions. At this point, Munakata appears, revealing that Arai had only shot Munakata with rubber bullets. As Munakata is about to have his bodyguards attack Kido, Daigo and Arai, the game's protagonists arrive via helicopter to confront their enemies. Akiyama decides to confront Arai; the two have a battle, which Akiyama ultimately wins.
After regaining consciousness following his loss, Arai helps subdue Munakata, and personally compliments Tanimura due to him being a cop of integrity. He accepts that he should stay in prison for his crimes. He initially opposes killing Munakata, wanting him to spend his life in prison, but Munakata is able to kill himself anyway. Arai goes on to serve time in prison for the murders of Ihara and Shibata.
Similar to Akiyama, Arai's fighting style is kick-based, although unlike Akiyama, Arai uses a few hand moves.
Arai starts the fight defensive, he has a triple kick sidestep counter that he uses both offensively and defensively. In his blue heat mode, Arai gains some new attacks. He has trip counter if you attempt to grab him. He can perform a heat action if you are on the ground, although this happens rarely. overall his attacks are still the same. He also gains a knockback attack that will break your combo and counter with a collapse attack, unless its blocked before the hit. In red heat, Arai still has the same moves from his blue Heat mode and gains even more attacks. His sidestep counter has an extension that will break guard. He then gains a running attack that hits high and low, causing the player to trip on low. He also gains the ability to perform a leg grab counter if you attack him. He will then try to attempt a grab that will go into a heat action that'll take a chunk of your health.