Duo comes across as a relaxed and laid-back kid with an easy charm and a ready sense of humour. Underneath, They've got more than a few issues and while they'll cultivate a lot of friendships, they'll be shallow to some degree.
Their reluctance to let people get closer stems from their certainty that they're ultimately a bad thing for people. Duo'd parents died, friends died, their guardians died, and when their powers were expressed, the first people they possessed died. When they talk about being Shinigami or the God of Death, they're only half-joking.
Duo is also averse to touch. Not that they don't love it, but although their control of their phasing power is good, they've got no idea how it works and do not want to violate people they care about by accidentally possessing them.
One good thing about Duo is that they're hard to honestly anger. Confuse, irritate, trouble -yes, but they don't hold grudges and aren't interested in harbouring bad feelings about other people.
Duo can phase into people and control their bodies, effectively possessing them for as long as they choose. Any damage the possessed person takes won't effect them and if the possessed person is killed, Duo is shunted out of their body unharmed (although also unprotected in potentially dangerous situations).
While he's possessing someone, he doesn't have access to their thoughts, knowledge or memories. This is probably some sort of unconscious defensive action on his part to keep himself firmly partitioned from the mind of the other when he's in someone's body. Evidence of this is that he has some mental defenses against psychic attacks. This is nothing a good telepath or empath couldn't break through, but much better than one would expect from an untrained teenager. It might be that he could eventually learn to sense memories and thoughts from the people he possesses, but thanks to some very firm moral teaching from his Jesuit guardian and the experience of possessing a lot of people on his journey to the school, it's not something he's interested in.
Duo's phasing power is something he's only used on humans and is likely limited to living things. It's also limited to merging with them, not passing through them on a whim.
Duo is skilled at stealth. To phase into people they have to be in physical contact, so approaching someone and then phasing into them unseen requires a lot of care and quiet.
They're also good at sports like basketball and soccer.
Anyone hanging around with Duo in their free time might find themselves dragged along as they go to junk shops, driveways on garbage days, etc. to find stuff they can use and/or tinker with. A lot of what they eventually owns will be things that they found and repaired, and they're exceptionally good at returning junk to working condition.
Duo can speak Spanish, English, read and write Latin and a little bit of Koine Greek.
Scavenging is one, but having a Jesuit priest for a mentor meant a good grounding in history, science and classic Western literature, so Duo's a voracious reader.
Duo’s parents, immigrants to El Salvador from Japan, died when they were young and their memories of them are foggy at best. After their death, for reasons of bureaucratic incompetency, Duo was mostly left to bounce around to a the few neighbours his parents had known, eventually spending more and more time running the streets of San Salvador with friends untitl yhye finally stayed on the street
Around the age of nine, Duo was scooped up and brought to an orphanage run by a Jesuit priest, Father Maxwell. At the orphanage, the Father singled Duo out as an especially bright child and began schooling them himself, giving Duo the foundations of a classical education. They became close and the relationship soon became something akin to a father/child relationship. Sister Helen, a Dominican nun who assisted in the running of the orphanage also took Duo under her wing and become a formative figure in Duo’s childhood.
Around the time Duo turned 16, gunmen attacked Father Maxwell and Sister Helen in the orphanage chapel. To this day, Duo doesn’t know if the attackers were simple criminals or if there were bigger political aims at play, but it was during this event that their powers first manifested. Seeing the gunmen threaten Father Maxwell, Duo leapt at the gunman, intending to knock him down, but instead they found themself merged with the man. While Duohe panicked, gunfire erupted, and in the chaos that ensued Father Maxwell and Sister Helen were killed and Duo’s host was mortally wounded. He were left by his partners to die, when he finally did, was Duo was shunted out of his body.
In the following weeks Duo lived in a state of panic, jumping from one person to the next to stay in hiding until one day they heard a summons from Professor Xavier and decided to make the trek north.
During the trip they honed his skills by finding hosts and using them as disguises as they traveled. Although they can’t access the memories or thoughts of his hosts, they did go through purses and wallets and eventually began to realize there was a moral issue to essentially stealing their bodies from them. They haven't sorted it out yet, but they know they know power is easy to abuse and would like to avoid that as much as possible. The trip also helped them sort out one other detail about themself. Living in a wide variety of bodies confirmed that gender wasn’t something they were really interested in applying to themself, so Duo considers themself non-binary (Tehy/them, he/him, and she/her are all acceptable pronouns for Duo).
Duo arrived on the mansion doorstep in their own body, full of curiosity about what it had in store for them and eager to leave their childhood in the past.