Tohru Adachi is a rookie detective in the Inaba police department and is a member of their investigation team for the murders taking place in Shin Megami Tensei: Persona 4. As Ryotaro Dojima's partner in the force, he is often clumsy and somewhat incompetent, but also serves as a voice of reason when his elder becomes especially brash with others. Although he bumps into the Investigation Team while they're conducting their own investigation on numerous occasions, he has a tendency to turn a blind eye towards their actions if he deems them to be for the greater good. His voice actor in the English version of the game is Johnny Yong Bosch, an anime industry veteran known prominently for his work in series such as Bleach and Power Rangers.
Should the player pursue the route leading either to the normal or true ending, the conclusion is eventually reached that Adachi is the actual murderer and mastermind behind the kidnappings instead of Taro Namatame. Indeed, while he initially tries to avoid being labelled as such when confronted with it, he eventually confirms that to be true. Stating that he essentially triggered them all for his own amusement, as well as vengeance for being transferred to the out-of-the-way Inaba department against his will, he is only directly responsible for the first two deaths, those of Mayumi Yamano's and Saki Konishi's, as well as the kidnapping of Mitsuo Kubo. The remainder of the incidents, however, occurred because he manipulated Taro Namatame into believing that bringing people to the Midnight Channel would save them from death, when in reality the opposite was true: people were dying because they were in the TV World.
After fleeing to the Midnight Channel himself, he attempts to bring that world's fog into the real world in an effort to transform all of humanity into Shadows. Justifying his efforts by saying it is what mankind actually desires, he reveals that bringing about the fog will improve people's lives since they long to be ignorant and becoming Shadows fulfills that wish. Although he brushes off the Investigation Team's search for truth as a worthless endeavor, he grudgingly follows their order to be brought to justice in the human world after losing to them in battle.
In the true ending route, Adachi sends a letter from prison which provides the initial spark which unravels who was truly manipulating the events in Inaba. Although the letter is cryptic in nature because of his inability to recall much pertinent information on his own, it is enough to ultimately point everyone in the direction of the dark goddess Izanami, who masqueraded as a mere gas station clerk at the very very beginning of the game. When confronted, Izanami openly admits that she is responsible for giving Adachi, as well as Yu Narukami and Taro Namatame, the power to enter the TV world, in addition to their ability to call upon Personas.
Prior to his assignment in Inaba, Adachi was a high-ranking elite who had been stationed at police central headquarters. Intelligent and ambitious, his academic achievements had enabled him to enter the force as an assistant inspector and ought to have placed him on a fast track of promotion. However, that advantage was stripped from him as the result of what he insisted was a small mistake (the nature of which is never explained), and as a result he was demoted and spirited away to Inaba in spring of 2011. Despite this, Adachi maintained his status as a detective, though he continued to resent his superiors for their decision and also his former colleagues for supporting it.
Soon after his arrival at Inaba and without his knowledge, Adachi was granted the power to enter into the TV World by the goddess Izanami, who had been posing as a gas station attendant. He was then partnered in Inaba's police force under Ryotaro Dojima, a senior officer who had never taken a promotional exam and whom he would not be given the opportunity to surpass. Bored out of his skull and effectively becalmed in his career, Adachi ceased to take his duties seriously and thereafter passed himself off as an incompetent.
At some point prior to the arrival of Yu Narukami, Adachi acted on rumors of the Midnight Channel he had heard both from Izanami and from others at the office. This led to the discovery of his goddess-granted powers, and filled him with a sense of excitement he had not felt since coming to Inaba.
On the first day of the game, while Dojima is off meeting his nephew at the Yasoinaba railway station, Adachi visited the Amagi Inn under the pretense of guarding Mayumi Yamano, a local reporter for whom he had developed feelings and would later that night kill when those feelings were not returned. While there, he spoke with one of the workers about the rumors of Yamano's affair with Council Secretary Taro Namatame, stating that fame could be rough, at that time choosing to believe that the allegations were false. He then made arrangements to meet with Yamano alone later on, in order to tell her "something important". Yamano's murder kickstarts the game, as she is the first victim to die in the TV World.
A running joke within the Persona 4 fan community pertains to the occasional appearances which Adachi makes at Junes, Inaba's local multi-purpose store akin to Wal-Mart. If the player approaches Adachi and then proceeds to talk to him, the dialogue has a tendency to reference the fact that he is in the store shopping for dinner. What gets parodied from these dialogues is the fact that Adachi has an odd love of cabbage in particular, and has a compulsion to get that on his shopping trips. Due to the seemingly random nature of picking that vegetable in particular to enjoy, the fan community has gone out of its way to make fun of the trait. As such, numerous fan art depictions of Adachi with cabbage can be found on the Internet, especially on Japanese fan art sites.