Maiza was born in the fictitious Neapolitan town of Lotto Valentino sometime in the late 1600s. He had a younger brother, Gerd (or Gretto in some translations) whom he cared for deeply. At some time during his youth (~1705) he was a delinquent, leader of a gang of upper-class boys with too much time on their hands, but by 1710 he had put this behind him and become an alchemist, despite the danger from suspicious religious authorities. He was ambitious and idealistic, and hoped to use knowledge gained through alchemy to better the world. Sometime before 1711, Maiza encountered an alchemist who had also known Maiza's grandfather-and had not aged since that time. The man claimed to be 300 years old and to have obtained immortality by summoning a demon that had taught him the coveted secret of the Elixir of Life. This alchemist taught Maiza the method by which he had summoned this entity. (The only info on this in translated canon is a rather vague description of Maiza. Also the guy who taught him may have been Nicolas Flamel-it's not directly stated in canon, but the time periods match up.)
In the year 1711, Maiza, his brother, and about 28 other alchemists decided to travel to Boston, in what would become the USA, together on board the ship Advena Avis. During this voyage, Maiza revealed the secret he'd learned from the 300-year-old man and proposed that the alchemists use it to summon the same entity. This suggestion was met with near-universal approval (Gerd Avaro had reservations, but did not publicly voice them) and the ritual was performed the very same night. The demon responded, and did indeed grant the assembled alchemists immortality; however, it decided to have a bit of fun with these clueless humans. Firstly, immortality came with certain conditions; immortals would be invulnerable to dying by normal means, but could kill each other as easily as by placing their right hand on the intended victim's head and willing it, whereupon the victim would be completely absorbed into the killer and the killer would gain all of the victim's memories. This was referred to as "eating" or "devouring" the individual. They also have the ability to place knowledge directly into each others' minds by a similar procedure (right hand on the recipient's head) and are incapable of using false names in each others' presence or establishing a permanent false identity. Secondly (and here's the kicker), although all of the alchemists were given Elixir to drink, the demon only gave one man the knowledge of how to make more of it: the man who had led the summoning ritual, Maiza Avaro. Naturally, things subsequently got rather messy. Maiza decided to restrict the knowledge to himself only, thinking that it was dangerous to risk any leakage of such a powerful secret and that the number of immortals in the world should not be allowed to increase too much. Most of the alchemists accepted his decision, but one man, Szilard Quates, objected vehemently and demanded that Maiza share the knowledge. That night (the night after the summoning) Maiza confided in his brother that he feared that Szilard would not yield and that Maiza would be forced to eat him to protect the secret. He admitted that he was beginning to regret summoning the demon in the first place, but that he still believed that the knowledge of immortality could be used to make the world a better place, by granting immortality to people who were truly dedicated to "bringing miracles to this world". To this end, he conferred half of the knowledge he had received from the demon upon his brother. Gerd objected, not wanting the responsibility, but Maiza insisted that there was no one else whom he trusted as thoroughly. Unbeknownst to either of them, Szilard was lurking about at the door and heard the entire exchange. In fact, Maiza's fears had been realized. Szilard's envy and hunger for knowledge had driven him insane and he had begun to sneak around the ship, eating whomever he came across, with the ultimate intention of eating Maiza and gaining his knowledge of the Elixir. Later that night, Maiza went to Szilard's room with the intention of eating him (still unaware that Szilard had already snapped, he was simply afraid that it would happen). However, when he attempted to do it, he realized that Szilard was not in his bed and that the worst had already come to pass. He returned to his own room to warn Gerd of the danger, only to discover that Szilard had already eaten him. He raced to find Szilard and avenge his brother, but in the resulting confrontation, which occurred on the deck of the ship, Szilard was knocked over the side. Realizing that Szilard would not have drowned and might still be a threat, the surviving immortals (13 had been killed) split up when they reached Boston and cut all ties with each other; if one knew any of the others' locations, then so would Szilard if he found them and ate them. As the one who had proposed the summoning, Maiza blamed himself for the entire affair and would struggle with the guilt for the next two centuries.
Now we take a flying leap from 1711 to 1930. Maiza is in New York, working as a bookkeeper for the Martillo crime family, a relatively small and weak organization with a speakeasy and maybe a couple of casinos. He has grown tired of life, but does not actively seek death. He's also found a sort of surrogate brother in the person of Firo Prochainezo, a young and dedicated member of the Martillo Family. Szilard is also in New York, running a top-secret brewing operation which has just succeeded, after 200-odd years of trial and error, in recreating the formula for the complete Elixir of Life from the half-formula that Szilard gained by eating Gerd. However, Szilard and his people he is totally mooching off of and intends to eat when they have outlived their usefulness associates lose control of the Elixir (two bottles). Through a long and convoluted series of events that is not strictly relevant to this application the bottles wind up in the possession of the Martillo Family, who mistake them for wine and proceed to drink the contents in celebration of Firo's initiation as an executive of the Family. And then Szilard shows up looking for it, and finds Maiza, and there's a big confrontation between the two of them, during which Szilard guns down all the executives of the Martillo Family, who knew nothing of immortality and had been trying to protect Maiza from this unknown assailant despite Maiza's urging them to flee. However, having all consumed the "celebratory wine", they promptly come back to life and demand to know what's going on. At a crucial moment, a servant of Szilard's whom he brought to assist him in recovering the "wine" betrays him (the specifics are irrelevant, but he's been treating her like garbage for the whole show and for quite a while before that, and her loyalty to him was based only on fear of him) and tells the new executive, Firo, the method of killing other immortals. Firo puts it to good use. Szilard's memories enable the young gangster to understand what has happened, and after seeing the others revive from getting shot there can be no doubt in Maiza's mind as to the situation. Maiza, who has remained relatively collected throughout the whole ordeal up to that point, breaks down at the thought of the Martillos facing the "curse" of immortality and asks Firo to eat him as well. Firo refuses, assuring his friend that it's really not that bad ("It's not as if we wanted to die!") and reminding Maiza of his importance both as a member of the organization and as a friend to many of its members. The assurance that Firo, who was now immortal and had (by way of Szilard) a complete understanding of the events of 1711, did not blame him for anything helped Maiza begin to move past some of his remaining guilt over these matters.
In 1931, Maiza was contacted by an acquaintance from 1711, the child-bodied immortal Czeslaw Meyer. Czes had experienced a terrible, traumatic betrayal after becoming immortal and had come to believe that no one could be trusted. He planned to eat Maiza upon meeting him in New York, but Maiza wanted only to help the "boy" recover from his trauma.
On the surface, Maiza is a very calm, good-natured, and easy-going individual. He is polite to everyone he meets, although his politeness may take on a sarcastic edge if he is angered or dealing with someone he dislikes. He is very self-controlled and can retain his calm even in the most trying of circumstances. Some have found his unwavering calm unnerving, as it strikes them as unnatural, and his nonthreatening demeanor has earned him ridicule from some of his fellow gangsters. Despite his outer warmth, there is always a sense of reserve about him; he is sociable and enjoys conversation, but prefers to listen rather than talk about himself. Even among his closest friends, he rarely expresses his deeper feelings or reveals much of his inner life.
Maiza is a kindhearted man and tends to put others before himself. He has no specific moral code, and follows what his conscience tells him in a given situation. He dislikes needless cruelty or conflict. Despite his involvement in a criminal organization, he tends to avoid violence, preferring to negotiate or (if necessary) intimidate his way out of a conflict, although he will fight if attacked. He is deeply loyal to and protective of his friends (he worries about the, and is always willing to offer advice or help if needed. He also seems to enjoy the feeling of close camaraderie with a group (the 1711 alchemists being another example) and this may have been part of the reason why he chose to join the Martillo family despite the violence of their business. He has often assumed a mentor role, which a Baccano fan other than myself described as "filling the Gerd-shaped hole in his life".
Due to a combination of simple world-weariness and the tragic events associated with its acquisition, Maiza has no love for immortality. He has come to regard it as an unmitigated curse, corrupting people, inflicting and prolonging suffering, and spoiling one's appreciation of life. He suffers tremendous guilt for having brought this "curse" into the world and for indirectly causing the aforementioned tragedy. (After 1930, his view of immortality begins to shift toward a more positive view, but the change is slow.) This guilt has some hold over him; he feels responsible whenever anyone suffers for reasons related to immortality, and his greatest fear is probably that he might at some time be responsible for another tragic event as he was in 1711; in 1930 he thought this fear had been realized. Despite this, he recognizes the futility of wallowing in regret and makes every effort to live in the present and make the best of his situation. He professes a strong belief that human beings have the power to overcome their personal sorrows through their own effort and will; this belief is most likely based in his own experience of learning to cope with guilt and overcome grief. His world-weariness has also instilled a bitter, cynical streak in him, which he usually does not express, as he prefers the optimism that seems to be native to him.
Let's see, anything else: Maiza has a lot of confidence in his own judgment, maybe a tad too much (200 years of life experience does tend to make one feel qualified to say a thing or two, disastrous mistakes of 1711 or none). He has a whimsical side and enjoys simple games and amusements, which he says bring him peace of mind. He's also got a bit of an inner philosopher. Due to his epic lifespan, he takes the long view/big picture on things. Not to mention, he has a strong distrust of anything that's supposed to surpass the limits of normal human experience (the Advena Avis was kind of a too-close-to-the-sun experience for him).
His immortality is defunct in Nautilus, as are the adjunct abilities that come with it. More info can be found on his journal.