In 2001, Redd White bribed Grossberg into giving him scandalous details about a spirit medium named Misty Fey, whose abilities had been called upon by the police to assist in the investigation of the DL-6 Incident. White anonymously disclosed this information, causing Fey to be publicly discredited and leave Kurain Village in disgrace. After discovering that the police were searching for the source of the leaked information, White turned on Grossberg, blackmailing him into doing whatever White liked lest he release to the public and the police what Grossberg had done. Grossberg's mistake continued to haunt him for the next 15 years, as he had not only left himself vulnerable to blackmail by the opportunistic White, but also inadvertently ruined the reputation of Misty Fey and pretty much the entire Fey clan.
Several years later, Mia Fey, Misty's daughter, joined the Grossberg Law Offices. Grossberg and his associate, Diego Armando, took it upon themselves to assist her and teach her the trade. Fey's first case in court ended in tragedy when the defendant, Terry Fawles, poisoned himself and died on the witness stand. Not long after, Armando was himself poisoned and left in a comatose state.
A few months later, Fey returned to the courtroom in the defense of art student Phoenix Wright, which initially surprised Grossberg. Wright refused to cooperate with Fey on the witness stand, and Grossberg gave up, remarking, "I knew that boy was guilty the first time I saw him." However, Fey convinced him to trust her and, with his cooperation, the defense managed to prevail. Grossberg later realized that Fey's true goal was to bring Dahlia Hawthorne to justice, as she had poisoned Armando. With Grossberg's help, Mia managed to prove that Wright was innocent and that Hawthorne was the one who had murdered the victim, Doug Swallow.
In the end, Grossberg was impressed with Fey's dedication to her client, and realized that such devotion to one's client was what being a lawyer was all about, a concept he believed had been lost among his contemporaries.
Mia Fey eventually left the Grossberg Law Offices and created her own law firm, Fey and Co. Law Offices. However, she would later be murdered by Redd White, and Mia's younger sister Maya would be arrested for the crime. When Wright came to Grossberg's law office requesting his help, Grossberg flat-out rejected, stating that this was one case he couldn't take. Wright would ultimately take the case himself.
Later, Wright returned, needing information. Grossberg told him about White and his company, Bluecorp. Wright also noticed that a grand painting of Grossberg's had suddenly disappeared. He saw it again when he went to Bluecorp, and questioned Grossberg about it when he returned. Grossberg confessed to the blackmailing that White had subjected him into for 15 years, which was why he could not take Maya's case.
Later that year, Wright returned to Grossberg Law Offices to request information about the DL-6 Incident. Robert Hammond, one of Grossberg's employees, had been murdered. Grossberg told them that Hammond had been the defense attorney in the DL-6 Incident. Wright returned again, asking about a lawyer named Gregory Edgeworth. Grossberg revealed to Wright that Gregory Edgeworth had stricken prosecutor Manfred von Karma with the only penalty of his career. Grossberg also told Wright about Misty Fey's involvement in the DL-6 Incident. With Grossberg's help, Wright was able to pin down Hammond's killer and uncover the truth behind the DL-6 Incident.
Grossberg is very strict and tends to speak nostalgically of his past. He has a habit of loudly and exaggeratedly clearing his throat to draw someone's attention. Grossberg also suffers from hemorrhoids and frequently complains about them. He appears to be fond of his past employees even after years of seeing them. This sentiment extends to their relatives, as shown by the kindness he shows Maya Fey, although this may have been at least partially influenced by guilt for his role in Misty Fey's fall. In a sense, Grossberg represents an old generation of veteran lawyers who have perhaps lost sight of their trade's true purpose and become tainted with corruption. After seeing Mia Fey's first victory, he remarked on her ability to trust in her client.
His Japanese name, "Soranosuke Hoshikage" (星影宇宙ノ介), is both astrology-themed and a reference to two Japanese mystery novels; "Soranosuke" (宇宙ノ介, "star boy" or "boy of the stars") comes from a reference made by the main character in Soji Shimada's début mystery novel "The Tokyo Zodiac Murders", while "Hoshikage" (星影, "starlight" or "star shadow") comes from the name of a detective in a Tetsuya Ayukawa novel.
His surname, "Grossberg", originates from the German words "groß" (large) and "berg" (mountain).
His French name, "Rosenberg", is a Germanic-language family name and toponym (a place name). Its principal meaning is "mountain of roses", from "rose" + "berg". However, as a toponym, in some locations it may have originally meant "red mountain" or simply "red hill", from "rot" + "berg".
Marvin Grossberg was only given four animated sprites in Phoenix Wright: Ace Attorney because the developers could not fit any more into the game due to memory restrictions at the time. His reappearance in Phoenix Wright: Ace Attorney: Trials and Tribulations keeps the same number of forward-facing sprites (reused from his previous appearance, albeit wearing a red suit; the reason for this color change was because his usual brown one blended in too much with the courtroom colors), but does give him some new side-on sprites for his role as Mia Fey's co-counsel (albeit with the only two facial expressions he normally has), as well as making him slightly thinner.
Grossberg is, to date, the oldest character to act as a protagonist's co-counsel.
His hemorrhoid affliction was added by Shu Takumi on a fluke. He has admitted that if he had written the character on a different day, then he might not have been in the mindset to give him that quirk.
↑Grossberg: It was 15 years ago now...I received a request from a medium. A spirit medium. Phoenix: (A medium...?) Grossberg: Her name was Misty Fey. Phoenix: (Fey!) Grossberg: Indeed. She was Mia's mother. She had been investigating a murder at the bequest of the police. And... she failed. As a result, the police called her a fraud. Phoenix: (This is what Maya was talking about the other day!) Grossberg: I did all I could for her, and in the end, cleared her of wrongdoing. Phoenix Wright: Ace Attorney.Capcom. Episode: Turnabout Sisters (in English). 2005.