I have a lot of theories about Thiago, and his role in the Sentinels storyline.
Here is one that I've been working on for a while, incorporating some of the recent revelations we've seen in the Sentinels Tactics Kickstarter.
I've wondered for a while if Thiago was an orphan (I don't know if that is confirmed, or was just an idea I had), and speculated about his biological parentage (IE, the Operative being his mother)
Thiago was left at a religious orphanage when he was just a baby. Rook city being what it is, there were more than a few orphans, and caring was in precious short supply. As he grew, Thiago became the big brother of the group. He protected the younger kids, not by striking first or starting fights, but by talking with the bullies, finding out what was going on, and playing the role of peacemaker.
However, Rook city being an unforgiving place, there were always some who only wanted to use violence, and when cornered or given no options, Thiago proved to be adept at fighting.
Flash forward a few years.
Spite is on the prowl, and there is a lot of fear in the orphanage. One of the priests has gone missing, and Thiago (who volunteered at the soup kitchen with the priest) remembered seeing a man in a hoodie talking to him earlier. Thiago has been following the adventures of the heroes around the world, and has started venturing out on his own at night to do what he can against the darkness.
Kind of like the neighborhood watch- just being around in a Legacy shirt staring miscreants down(and there's something about the way he stares, like he's not afraid of anything, that makes them think of a terrifying figure from the underworld...).
Spite sees him, and decides that he'll make an ideal victim. So he approaches and spends some time gaining the kid's trust. He senses that the kid is wary, and goes to raid a lab for the night, leaving Thiago for later (the best kills are betrayals of people who trust him, another theory I’ve had for a while).
The Wraith has been watching from the shadows, and swoops in to rescue the boy.
But seeing her, he wants none of it. He's willing to be bait. He wants to draw Spite out and get him into position for the Heroes to take him down without collateral damage or innocent bystanders.
The Wraith is troubled by this, but Thiago doesn't back down. Its the right thing to do, and the best way for him to help. She allows it, seeing that Thiago isn’t going to back down one bit, and would do it anyway if she tried to prevent it.
The plan succeeds and Spite goes down (and is imprisoned in the Block), after a terrible fight.
But the Wraith stays in contact with Thiago afterwards.
Flash forward a few years.
Thiago continues to grow. He's had a few scrapes on the streets (including a rescue by Mainstay). Not that he was afraid. He could have taken them.
Maia Montgomery has been keeping tabs on him, and decides to adopt him. She hones his skills, both mental and physical. He doesn't agree with her shadowy ways, and that is the source of a lot of arguments between them. However, he does love and admire her, and over the years his arguments start to sound more reasonable to her. Maybe it is time for the Wraith to unmask- Several of her enemies already know her identity, and maybe she can do more good bare-faced to the world.
So, she turns over control of Montgomery industries to Thiago, who is proving to be an extremely astute student. She's bought up Pike's old businesses, and Rook city is starting to revitalize (though it still has problems!)
And Thiago starts standing up for good, justice, and the light as.... The Chairman.