Bashar came from a troubled family; when he was young, his father was killed piloting an aircraft while protecting Metallurgic International's claim to a mine. Because of the loss of his father and the fact that his mother was out of regular work due to an injury inflicted by a malfunctioning robot at her workplace, Bashar and his mother were forced into poverty and he developed a disdain for corporations and authority in general.
Sometime during his childhood or early adolescence, his mother remarried to a high-ranking executive of Metallurgic International named Wyatt Mahante, rescuing their family from poverty in the process. Despite this, Bashar hated Wyatt for his connection to Metallurgic and because he felt Wyatt was a poor replacement for his biological father.
Becoming a delinquent, Bashar fell into vandalism and drug use, recounting in one Vantage how he wrecked a museum with a friend simply for the fun of it. After overdosing on a highly potent drug at a concert, he nearly died when security bots tazed him and was sent to hospital. After this, he resolved to improve his ways for the sake of his mother and went to rehab.
Bashar developed an interest in astronomy as he became an adult, eventually getting a degree in the field and getting a job designing space exploration drones. His mother approved, and was proud of his achievements, stating that Bashar had written their family's name across the stars.
Unfortunately, this was not to last. Due to being slowly poisoned by chemicals from her old job as a Metallurgic International robot technician in a mine, Bashar's mother was slowly dying. She did not tell her son, as she didn't want to pile more problems on him. By the time Bashar found out, it was too late; she had slipped into a coma, and died after she was taken off of life support.
The death of his mother had a profound and lasting effect on Bashar; he went effectively AWOL from his job and sank back into drugs (though, out of respect for his mother, never anything as hard as what he overdosed on). After an intervention by a colleague, he went into psychiatric rehab to deal with his mother's death, after which he returned to work. However, due to his effectively abandoning his old job, he was reassigned to a 'dead-end position' designed to indirectly push him out of his job--the Vantage project.
Discovery of Project Zero Dawn
Bashar was working on the Vantage project around the time that the Faro Plague went out of control. While being interviewed by Samina Ebadji about Vantage technology (presumably judging its viability for use in the APOLLO project), he noticed Brad Andac, an old colleague of his, was present at the interview. Curious as to the purpose of the interview, Bashar broke into Brad's files, and through his journals found out about the oncoming apocalypse and Project Zero Dawn.
Musing that at least his mother wasn't alive to see the end of the world, Bashar initially contemplated committing suicide, intending to do so by overdosing on a small fortune's worth of drugs in Wyatt's cottage. Before he went through with it, however, he decided to go for a walk, and after seeing the innate beauty in a world soon to be utterly destroyed, was inspired instead to spend what time he had left leaving records behind for any future life that might exist on Earth.
Leaving the Vantages
To this end, he burned the drugs he was intending to kill himself with (promising to his mother that however he died, he would die clean) and rented a luxury car. He first traveled to FAS, where he took twelve "spikes" from the Vantage project. These spikes, designed with Eternity chips, were made to hold data for around 10,000 years; perfect for what he intended to do with them.
Traveling around what was the Colorado-Utah area, he went on what he called the "Apocashitstorm Tour" to various spots that had an impact on him during his life--the waterfalls near the mine where his mother was injured, the museum that he vandalized, and the FAS headquarters, among others--and drove the Vantage spikes into the ground. On the spikes, he left an image of the locations as they appeared during the Old Ones' days, along with an audio message and pages of a written journal addressed to his mother, reminiscing about their lives leading up to Zero Day.
In the final Vantage journal, left outside the Project Zero Dawn development labs, he expressed a rather nihilistic view that for all his and humanity's accomplishments, it all came to nothing, as his career--and humanity as a whole--was effectively over, and compared to the scale of the universe, they never made much of a difference. Despite this, he maintained that the whole reason for his leaving the Vantage spikes was to tell not the story of humanity and how it ended, but a different story - not the big one written across the stars that his mother hoped he would write, but his story; of a boy who loved his mother very much.
Even though he doubted that anyone--or anything--would ever read it, he then signed his name along with his mother's, father's, and stepfather's as the final words of the journal.
The final fate of Bashar Mati is not known, though he likely perished along with the majority of humanity as the Faro Plague consumed all life on Earth.