He goes on a mission through time with childhood friend Neon Tsukiyumi after demons escape the family's imprisonment and rewrite history through a series of murders.

Other returning team members included producer Yoshinori Kitase, artist Yusaku Nakaaki, and composer Masashi Hamauzu.

The team used tools supplied by CRI Middleware to both fit all their planned content on the DS cartridge and incorporate Hamauzu's music with minimal reduction in quality.

First announced in March 2008, it was promoted using an episodic radio drama released through the game's website, and a series of commercials blending gameplay footage with anime sequences created by Sanzigen.

Reception of the game in Japan and its limited Western reviews has been generally positive, with praise going to its battle system and investigation mechanics.

[5] When enough clues are assembled, Sigma can trigger the Superdeduction system, his mental image of the case represented with a grid containing black nodes symbolizing key aspects of the mystery.

The clue medals must be arranged around the black nodes, unlocking a perceived sequence of events and allowing Sigma to reach a solution to the mystery using an inference to fill in blanks.

[2][10] The entire scenario takes place in the Kurogami family mansion, which changes depending on the time period and current events.

[2][7][11] Sigma is accosted one day by the mysterious Man in Black, and upon returning home finds the world changed for the worse and no-one recognising him except for Neon.

Using his power and the Great Clock, Sigma travels with Neon into the past, solving the murder of a household member, which is also being investigated by different versions of Rin Yukiha.

[9][12] Hiroki Chiba acted as director and writer, Yoshinori Kitase was producer, and Yusaku Nakaaki was the project's lead artist.

[13][14] Production took roughly two years, said to be long for a DS title, and had a fairly small staff number and budget compared to the company's work on the Final Fantasy series.

[15] Chiba enjoyed Shōwa period mystery writers such as Seishi Yokomizo and Rampo Edogawa, and wanted to create a game using that style.

[15] The cast included Daisuke Ono (Sigma), Aya Hirano (Neon), Tetsu Inada (Dixon), Mamiko Noto (Christie), and Jun Fukuyama (Rin Yukiha).

As production advanced, the team saw the ROM space was insufficient on its own, with Chiba fearing they would need to both cut the game down and reduce sound quality to fit it on the cartridge.

File Magic, which Chiba heard of by chance, allowed the cutscene illustrations to be compressed with a minimal reduction in quality.

[15] Chiba decided upon the uncommon book-style presentation to both reference his inspirations, and give players an easier time controlling the game.

[9][18] The gameplay systems were chosen so that players could advance through the battles and story sections regardless of whether they found the correct solution to a mystery, though bonuses were given for success.

[15] The gameplay mechanics ended up being quite complicated, with Chiba earning a reputation during development of retaining as many of his "cool ideas" as possible rather than cutting anything.

Having previously worked in a supporting role on Dirge of Cerberus and Crisis Core: Final Fantasy VII, Sigma Harmonics was his first job as lead artist.

[15][20][21] For his work, Hamauzu drew inspiration from the game's design document, its gameplay and narrative theme of rewriting the past, and Nakaaki's character illustrations.

[20] It was performed by Neon's voice actress Aya Hirano, composed and arranged by Shinya Saito, with lyrics by Aki Hata.

[32] A strategy guide was released by Enterbrain in September;[34] and a guidebook with behind-the-scenes material was published by ASCII Media Works in October.

[39] Sigma Harmonics remains unreleased outside Japan, a fact attributed to its heavy use of Japanese pop culture references.

[43] As of September 2008, the game has sold 70,000 units in Japan, ranking among Square Enix's best-selling titles for that year from new IPs.

[3] Jeremy Parish, writing for 1Up.com, positively compared the game to The World Ends with You and said that its blending of genres gave Sigma Harmonics "a distinctly unique feel".

[4] Joystiq's Sachi Coxon, giving praise to the gameplay mechanics in an article shortly after release, called the game "one to keep an eye on" for genre fans.

[1] Japanese gaming magazine Famitsu praised its gameplay mechanics as standing out from other RPGs at the time, though one reviewer found the process of collection information for deductions tedious.

[41] Michael Baker of RPGamer gave praise to the premise, soundtrack and battle system; his main faults were the lack of diversity in locations and the game's non-standard presentation using the DS's book mode.