Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: While no cohesive plot is apparent from the vignette style of the trailer, it can be pieced together that Batman has been murdered, and his killer remains at large. Dick Grayson is long since retired from his superhero days and raising a family with to his wife Barbara Gordon. After his former mentor's death, however, he decides to resume his crime-fighting days as Robin. Remarkably, Grayson does not take up the Nightwing identity. The filmmakers said they chose this because many people outside the comic book community are unfamiliar with Nightwing and they wanted to appeal to a wider audience.Commissioner Gordon is aware of Grayson's secret identity and assists him by supplying official documents. In addition, Gordon provides the voiceover narration at the beginning of the trailer. The head of the investigation into Batman's death is indicated to be Chief O'Hara, a character from the 1960's Batman TV series, who apparently also knows Grayson's identity (noting that Grayson's "crimefighting days are over") His role is suspicious since he strongly wants Grayson to not become involved, even to the point of aligning with Selina Kyle/Catwoman to eliminate Robin and shouting at reporter Clark Kent that he wants "him [presumably Grayson] out of the equation!" O'Hara is also seen rolling up his sleeves, preparing to assault an angry captive Gordon.

Grayson is aware of Superman's secret identity; he addresses him as "Clark". Superman apparently is also motivated (obviously from O'Hara) to discourage Grayson's return to crimefighting and three angry confrontations between the characters are shown, in and out of costume. Grayson is also angered to violence by the sight of a Superman comic book, suggesting a strongly negative history between the two. Other comic books also appear of characters from the film, including Wonder Woman and Catwoman. Fiorella used his own comic book collection for this scene.
Who was Dick Grayson's mentor?