Not much is known about the mysterious man except that he fights on the side of the Empire for his own personal reasons. However, he does aid the Order of Heroes as he warns them in Chapter 9 about Veronica's attack on their summoning grounds. He also seems to know something about Alfonse's missing friend, Zacharias, but refuses to say anything more before disappearing.
In Xenologue 1, he asks Veronica whether she feels as if someone is controlling her actions, so which she replies that sometimes the word "kill" pops up in her head, and then she wants to do so.
In Paralogue 3, he is reading through the tome of Bramimond at the World of Blazing, learning that the royalty of a certain nation has the blood of a wicked god, which uses the royalty as puppets, as they are unable to fight the god's influence; the text states this god is known by many names and that its true name has never been written until that point, but the name is not there, much to his frustration. He then proceeds to acquiring some information from that world, as it is the home of the Dragon's Gate, a portal to other worlds. He says that he must also find information on something of his, but this is left unsaid. At the end of the Paralogue, Nino gives Alfonse a hand-written letter from Zacharias, where it is stated that Bruno is the true villain, not Veronica.
In Chapter 10, which was added to the game later, he identifies himself as "Bruno", the prince of the Emblian Empire's imperial family, and an older brother to princess Veronica that has rarely been seen by the public. He also states that he does not want Veronica to conquer Askr, and gain the all of the glory.
In Chapter 11, Bruno sees Alfonse, Sharena, and Anna trying to call Zacharias while performing the rite and tells them that they're wasting their time, as the rite has failed to contact Zacharias. He then tells them that he killed Zacharias, leaving Alfonse shocked. Before he leaves, Bruno tells Alfonse and Sharena that they are next.
In Chapter 12, Bruno continues to remind Alfonse, Sharena, and Anna of what he told them in Chapter 11 about how he killed Zacharias. He even presented Zacharias's lance before he fought the group himself. While Alfonse was considering to execute Bruno, the Summoner tells Alfonse about what they experienced at the end of Chapter 10 when Zacharias helped them flee. Theorizing that they may have actually been speaking to Zacharias, Alfonse asks Bruno if he is actually Zacharias.