Marcello’s personality is depicted as rather controlling. His half-brother, Angelo, is a large reason as to why Marcello appears so cold during interactions with the party. Being the illegitimate son of one of his housemaids, he was thrown out of the house at a young age and taken in by Abbot Francisco. (As was the norm for orphans at the time.) In the 3DS version of the game, Marcello and Angelo alike are pictured at different times with the same love interest as the temporary playable character, Xiroara.
Marcello was the son of a wealthy, but cruel man and one of his maids. His father found that his wife would not bear a child and so had Marcello with his maid. While Marcello was still a boy, his father did manage to have a son with his wife, naming him Angelo. Marcello and his mother were thrown out of the house with nowhere to go and nothing to their names.
Not much later his mother died leaving him alone. Fortunately for Marcello, a leader of a group of monks at Maella Abbey, Abbot Francisco, took him in and offered for him to be a Templar Knight, his own guard. Marcello studied and trained hard for years at the Abbey and became the top young student. He later learned that his father and his household had died of a plague, all except Angelo. A few days later as Marcello was walking through the Abbey, he met a small boy who had just arrived. He talked kindly to the boy until he learned the boy's name. Angelo. Marcello, angered that the boy who had caused him to be thrown out of his home had been welcomed into the Abbey, walked away angrily as the Abbot appeared.
Marcello only grew to hate Angelo more as his half-brother took no care in his studies as he grew and only began to become a gambler and ladies' man, sneaking to the nearby town of Simpleton whenever he pleased. His hatred for Angelo only increased his bitterness and several sinister ambitions from deep down within him. Despite this, Marcello still became the Captain of the Templars and was still under the watchful eye of Abbot Francisco.
Marcello spent a few years as the Templar Captain watching Angelo and being under the watch of Abbot Francisco. The abbey is soon visited by a jester, Dhoulmagus with an eerie aura. The jester did not stay for long, leaving before night had even come. After this visit, Marcello tightened the guard so strangers would have limited access to the Abbey. A few days later, a group of travellers appear at the Abbey. Two Templars guarding the door that leads to to the sleeping quarters threaten them to leave when Marcello intervenes from a large window just above them. He reprimands the guards for being rude and sullying the reputation of the Templars.
The guards kneel and apologize to Marcello. Marcello, in turn, apologizes for his guards' behavior, saying they have had a problem with strangers as of late, but also saying that that was none of their business. He asks them to leave and warns them that his men are ill at ease and he might not be able to stop them again. The party then leaves and heads to Simpleton, meeting Angelo while there.
When Angelo returns from Simpleton, having gotten into a bar fight for cheating at poker, Marcello takes him down to the Torture Room to lecture him. Marcello asks him how much more shame he intends to bring to the Abbey and saying he is like a curse, and that if he hadn't been born, it would have spared everyone a lot of misery. After he is done insulting him, Marcello sentences him to a term of penitence, stripping him of his right to leave the Abbey, and threatens to expel him from the Order if he crossed him again. Ending the conversation, Marcello sends him back to the quarters.
Meanwhile, the group, who had come to return the ring that Angelo had given to Jessica, hears the whole conversation and are later asked by Angelo to take a secret passage through the Ruined Abbey to check if the Abbot is alright. Angelo states that he felt a sinister presence in the air and that the Abbot's life may be in danger. The party does what Angelo tells them and enter the abbey again through a trapdoor gravestone behind the Abbot's residence. They enter the building to see many unconscious guards and go to the Abbot's room where he is sleeping. Dhoulmagus is there, about to kill the Abbot, but he disappears when he sees them.
Marcello and the Templars arrive to encounter the party and a confused Abbot. Marcello kneels next to the Abbot's bed as Francisco asks him what was going on. Marcello tells him that there had been a breech of security and that some of the guards had been seriously wounded. Marcello says that he had come immediately fearing that he was in trouble, and caught the party. The Abbot protests, stating that they were no criminals. Marcello argues that they were caught red-handed, but Francisco simply tells him to look into their eyes and see that that are innocent. Marcello finally consents, but says that he must find out what they were lurking in his residence so late at night. The Abbot tells Marcello that he could question them, and Marcello thanks him.
Marcello is still not convinced, however, that they are not the attackers. He takes them down to the Interrogation Room, which is connected to the Torture Room. In the room, the party insists they have done nothing wrong, telling him that Angelo had told them to check up on the Abbot. Marcello tells them that he is not as forgiving as the Abbot and will not let them off the hook without any evidence. Angelo then enters the room, having been summoned by Marcello. Marcello asks him if he had permitted them inside the Abbey, presenting Angelo's ring, which he had found with the party. Unexpectedly, Angelo chuckles, taking the ring and stating, relieved, that it had finally turned up. He tells Marcello that someone had stolen it from him at the pub, and that they are finally going to learn that crime doesn't pay. Angelo then leaves, saying that he can be found in his chambers. Marcello tries to stop him, but Angelo doesn't listen. Marcello, saying that he will deal with him later, turns his attention back to the party.
Angered that so much of his time had been wasted, Marcello tells the party to confess. Once again they claim they did not do anything. Then another Templar appeared, with a little green monster in his possession, King Trode. The monster then turns to the party, calling their names and asking them what they were doing there. Marcello laughs, picking up Trode by the cloak, and berats the party for being friends with a monster. He presents his theory that the monster had ordered them to kill the Abbot so people would question their faith and send them into a confusion. Then they would move in and take control. He then orders the Templars to take them into a cell, so that he could torture them in the morning.
However, with the help of Angelo, the party escapes and leaves the Abbey through a secret passageway in the Torture Room. This was the least of Marcello's worries, as the jester returned, wreaking even more havoc then he had before. Dhoulmagus forces his way to the Abbot's residence, pursued by Marcello and the Templars. The jester sets fire to the bridge leading to the Abbot's residence, slowing down the pursuit, but Marcello and a few more Templars get through in time. Meanwhile, Angelo and the party see the bridge burning from nearby and rush to the Abbey to help. They cross the bridge just before it collapses.
Up in the Abbot's chambers, Marcello, his Templars already down, stands between Dhoulmagus and the Abbot. He is no match for Dhoulmagus and is thrown against the wall by Dhoulmagus. Angelo arrives by Marcello's side. Marcello orders Angelo to get the Abbot to safety, just as Dhoulmagus waves his sceptre again, throwing the two across the room. The Abbot qualms Marcello's worries, saying he has given himself to the Goddess, and his life was in her hands. Dhoulmagus drifts closer to Francisco, preparing to kill him, when suddenly Trode burst into the room. Dhoulmagus stopps and mock-bows to Trode, making fun of him. Trode demands that Dhoulmagus turn him and his daughter, Medea, back to normal. Dhoulmagus instead throws the scepter at Trode. Trode is saved, however as Abbot Francisco dives in the way being stabbed clean through by the sceptre. Dhoulmagus levitates the sceptre back to his hand as Abbot Francisco dies. Dhoulmagus mocks them once more then retreats, flying through the window, his laugh echoing as he disappears.
The Abbot's funeral is held the same night, the rain making everything even more gloomy. Marcello, who had managed to survive, except for a slightly bandaged head, explains to everyone what had happened the night before, and clears the party of any suspicion. And thus, Marcello becomes the new Abbot.
In the morning, Marcello summons Angelo, Trode, and the party to his quarters. Marcello apologizes to the party for doubting their innocence, King Trode having explained the entire situation they were in. Dhoulmagus stealing the sceptre, cursing the king and princess, and turning Trodain into ruins. Since he could not leave the Abbey himself, being the new Abbot and being very busy, Marcello asks the party if they would allow Angelo to aid them on their quest. Marcello tells Angelo that he was the only Templar who did not have any duties to fulfill, being the only one with an open schedule. Angelo claims that there was no need to beat around the bush that he was simply useless. Angelo and the party still agree, however, and Marcello gives them a map of the world, considering it a peace offering (the scene is omitted on the 3DS Version as the party starts with a world map), and wishes them well on their journey.
After Marcello becomes the abbot, he quickly rises through the ranks of the church, and becomes the third most powerful man in it. After the party saves the Lord High Priest from being killed by a possessed Sir Leopold, Marcello accuses High Priest Rollo - the next in line to become Lord High Priest, of hiring the party to kill the Lord High Priest - an obvious lie, as he knows the party - and has them sent to Purgatory Island. He finds the scepter, and, although there is no proof, it is heavily implied that he killed the Lord High Priest, as the story is that the Lord High Priest 'fell' from his mansion on the mountain, but he apparently had a wound that looked like he had been stabbed through the chest. He then became Lord High Priest himself. This worries the party, who assume he has been possessed by Rhapthorne and decide to confront him the Holy Isle of Neos, where Marcello is being sworn in as the new Lord High Priest. In fact, he managed to resist Rhapthorne's power, despite the seals being broken, by stabbing his own arm to prevent Rhapthorne from possessing his body. However, when the party defeats him, he loses control and Rhapthorne possesses him for long enough to throw the scepter into the Goddess Statue, where Rhapthorne's body and the Black Citadel have been sealed. Afterwards, Marcello is found by Angelo grasping to the side of a precipice above the deep crater from which the Black Citadel has just risen. Resigned to die, Angelo saves Marcello nonetheless. In so doing, he says, "No, I won't let you die. You will go on living, knowing that the brother you despised your whole life took pity on you." Marcello swears to make Angelo pay for saving his life, but Angelo doesn't care. Marcello gives up his templar ring to Angelo and disappears, never to return again.
He is later seen again by the group. (Nintendo 3DS version only) Not too long after fight with Marcello, Jessica receives a vision, prompting her and the others to return to the Dragon Graveyard. They do so and find another projection message from the Ancient Sage Golding involving Rhapthorne's ultimate servant, Juggerwroth. The party returns to the Wild Lands to find his lair, unaware that they have been followed since the desert by an injured Marcello (except for Princess Medea, who reveals later on in a dream that she saw Marcello keeling over in pain several times while following them to the Wild Lands, suspecting that his injuries from Neos didn't heal).
The party encounters and apparently defeats Juggerwroth, only to find that he was toying with them and sets up a spell to crush them with their own weight until Marcello launches a surprise attack. Though Juggerwroth powers up to his full extent, Marcello and the others are able to defeat him.
Marcello then takes his leave, claiming that he only aided them to repay his debt towards Angelo, who thanks his older brother, prompting the former to smile briefly.