The game is a crossover that combines gameplay, narrative, and aesthetic elements from Atlus's Shin Megami Tensei and Nintendo's Fire Emblem series.

Taking place in modern-day Tokyo and featuring real-life locations such as Shibuya and Harajuku, the game centers around hostile beings known as Mirages who seek to harvest energy known as Performa from humans and are responsible for several disappearances.

The Wii U GamePad displays an in-game social app known as Topic where players can receive messages from other characters, keep updated on available Unity fusions, and view a map of each area.

[3] The game features a turn-based combat system in which party members and enemy Mirages take turns to perform actions such as attacking, casting spells, and using items.

Performing well in battles, such as achieving quick victories or successful Session Attacks, increases party members' Stage Level, which unlock Performa for Radiant Skills and opens up new side stories.

The main casts are employees and associates of Fortuna Entertainment, a talent agency that secretly acts as a hub and recruiting organization for Mirage Masters.

The group are aided by Maiko Shimazaki,[j] a former model who is the head of Fortuna Entertainment;[k] and Barry Goodman,[l] a strict instructor from overseas with a love of otaku culture.

Tiki was banished to the Idolosphere, and saved Yashiro and Tsubasa from an attempted performance of the Opera of Shadows five years earlier at the cost of her memory.

Alongside the main narrative are side stories involving various members of Fortuna Entertainment, whom Itsuki can help as he learns from working in various sectors of the agency's business.

These quests' storylines include Tsubasa working to create her idol persona; Touma's wish to become an inspirational actor; Kiria's discomfort with her softer side, which clashes with her professional identity; Eleanora's attempts to polish her abilities as an actress in preparation for a hoped-for Hollywood career; Mamori pushing to expand her professional range beyond her cooking host role; Yashiro breaking out of his self-imposed personal and professional isolation, culminating in a battle with his father's spirit; the often-tipsy Maiko's efforts to maintain Fortuna's public and private functions; Barry's antics and his growing insecurity after losing his Mirage Master abilities; and Tiki wanting the chance to experience the real world.

If all the side stories are completed, a post-credits scene is unlocked where Maiko returns to modelling, and Itsuki is unanimously chosen as the new manager of Fortuna Entertainment.

Hirata was originally the project's sole director, but seven months after the game's public announcement, Ishida was brought on to co-direct so the amount of work needed for the title could be managed.

Their first concept turned out to be a grid-based strategy game with strategic positioning elements similar to Fire Emblem, but they were eventually convinced by Yamagami and staff from Intelligent Systems to keep within their skill set rather than attempting anything radically different.

The final product was visualized as a game only Atlus could make, incorporating elements from both series while standing as an original title that could be enjoyed by fans of both Shin Megami Tensei and Fire Emblem.

[20] The inclusion of a sharp symbol in the title, which represents a semitone increase in the pitch of a note in musical notation, denoted the game's status as a unique hybrid of the two series.

When going through how to help incorporate Fire Emblem characters within the "Atlus mythos", they decided to use a concept from Japanese Shamanism called the "kami oroshi", which stands for a deity possessing and communicating through a priest or ritual dancer.

[17] After Atlus was made primary developer, the story was set in modern-day Tokyo and written around a group of young people facing challenges together.

In the end, it was decided to make them summoned helpers similar to the demons of Shin Megami Tensei, drawn into the real world from their Idolosphere dimension.

They originally wanted to mimic the chosen areas of Tokyo very closely, but they needed to consider how this might make travel times between locations overly long for players: this was done by incrementally reducing the scale of environments while keeping the scenery and aesthetic intact.

The planned style, which would have vocal themes performed like music videos or sample trailers, needed a level of professionalism the team felt only Avex could provide.

After forming his team, Aburai worked to analyze the personalities and traits of the main cast to determine how their performances would play out, in addition to other factors such as their professions' impact on their portrayal.

[32] Ayane Sakura voices Eleanora, who despite being nervous made an effort to portray the character in a bullish yet clumsy way, pushing to balance her forward manner with making her appealing for players.

[45] Yuichi Nakamura portrayed Barry as the group's mood maker,[46] while Maiko's actress Ami Koshimizu worked to make her character both aloof and charming.

[32] The game was announced in January 2013 as part of a Nintendo Direct presentation under the English working title Shin Megami Tensei X Fire Emblem, and as one of many planned collaboration projects with third-party developers for the Wii U.

In addition, Atlus produced a "Fortissimo Edition", containing a special box, an original artbook, a six-track CD release including "Reincarnation", and downloadable content (DLC) outfits for the playable characters.

[64] Staff later defended these changes as necessary adjustments based on the regions they would be releasing the game in, while working on keeping the core identity intact.

Nintendo later issued a formal apology and offered pre-order cancellation options to Japanese buyers for failing to provide information regarding the changes ahead of time.

[79] Chris Carter of Destructoid noted that its focus on Japanese culture and similarity to fringe anime such as Revolutionary Girl Utena might put some people off,[75] while Game Informer's Kimberley Wallace called the narrative both amusing and predictable.

[91] Daan Koopman of Nintendo World Report enjoyed the narrative due to its brisk pace and highlights of the Japanese entertainment industry,[93] while Polygon's Janine Hawkins positively compared the story to magical girl anime, calling it "a co-ed version of Sailor Moon for the Love Live!

[91] Koopman said the battle system "[hit] the sweet spot" between Persona and Fire Emblem in its use of gameplay elements from both series in combination with its unique features.