From Halopedia, the Halo wiki
Dahlia was born on Arcadia in 2542, eleven years after the planet was first attacked by the Covenant. She grew up in the agricultural settlement of Abaskun until the age of seven, when she and her family were forced to flee off-world when the Covenant returned in 2549 and fully glassed the planet. They moved from Arcadia to the Inner Colonies before eventually reaching the Sol system, spending three years in a UEG refugee camp on Mars. During this time the family was often accompanied by Danzer and Pha, whom Dahlia viewed as family.
Arrival on Carrow
After the Human-Covenant War, Dahlia and her family joined a group of colonists who planned to create a new settlement on the planet Carrow along the banks of the Astlehich River. Traveling there in an old freighter, they found that the Sangheili on the planet had established the city of Rak on the spot where they intended to settle. As such, they traveled into the Uldt Desert, establishing the settlement of Sandholm and attempting to carve out an existence on the world. While she had a hard time in the town, Dahlia could still find moments of leisure and fun, such as riding quad bikes in the Uldt and eating lunch on Signal Hill. 
Illness and leaving Sandholm
In late June of 2558, a viral hemorrhagic fever swept through Sandholm and devastated the town's population. Dahlia fell ill and slipped into a fever-induced coma on June 25, after which her parent used up their supply of antibiotics on her before falling ill themselves. She woke up on July 2nd, finding her parents in a dire state and attempting to care for them. Eventually, Dahlia ventured from her home into Sandholm, where she found Danzer burning Pha's body outside of their home. He told her about the damage the virus had done to the settlement and that the town's communications repeater had gone down. Hiking up Signal Hill to investigate, Dahlia found the repeater damaged beyond repair, leaving the colony isolated from the other humans on the planet. Knowing that she needed to get help quickly, Dahlia decided to travel to Masov Oasis to find human traders and use their radio to call for help. Danzer initially tried to dissuade her from doing this, as Masov is controlled by the Sangheili, but eventually relented and gave her access to a Mongoose quad bike and a M295 Designated Marksman Rifle. With these new supplies, Dahlia left Sandholm and traveled until the evening, when she stopped to rest in a boulder field. 
Arrival and escape from the Masov Oasis
Dahlia was awoken by a firefight between Jat and Ruha. Finding Ruha about to execute Jat, she was able to distract the former for long enough to allow the latter to kill his attacker with an energy sword. Dahlia's Mongoose was destroyed in the firefight, leaving her to have to ride the rest of the way in Jat's Spectre. Dahlia continued to treat Jat with great hostility while the Sangheili offered his life to her in exchange for her saving his life.
After arriving at Masov, Dahlia parted ways with Jat and went to where the human traders were located. Upon entering the depot, she was quickly captured by two Sangheili who had taken over the building and locked in a cell in another structure. She then met Paul des Hommes and Greta, who told her about the deteriorating situation between the humans and the Sangheili in the Uldt Desert. After this, Masov was attacked by a Sangheili death squad loyal to Thars 'Sarov, who used human weapons in a false flag attack. Two Sangheili removed the humans from their cell and began to take them outside to execute them, but were intercepted and killed by Jat. Dahlia accompanied the Sangheili and fled Sandholm while Paul and Greta returned to the depot to radio for help. They were chased by Thars's forces and their Spectre was eventually disabled, forcing them to make a last stand. They were able to wound and kill several Sangheili but were both eventually wounded and overpowered. They were then saved by the Surakan Militia Volunteers, who had been called in by Paul and Greta and went to examine the firefight. Dahlia initially welcomed their presence, but then one of the militia soldiers executed Jat. She yelled at them and tried to defend Jat's name, but was only chastised for viewing an alien as her friend. Dahlia then received medical attention and new equipment from the militia before returning to Sandholm, where the remaining residents were also helped by the militia. She was present when her father awoke from his illness and told him that she was prepared to defend Sandholm from a Sangheili attack.
Personality and traits
Dahlia harbored a deep hatred and resentment for the races of the Covenant, especially the Sangheili. This was fueled by the trauma she received from the attack on Arcadia and her other experiences as a refugee during the Human-Covenant War and the fact that her family's plans on Carrow were ruined by the established Sangheili presence. She still harbored these feelings after her experience with Jat, but she understood that the situation was more complicated than she had previously perceived and vowed to fight them should the situation arise. 
Dahlia is also somewhat tall, as she observed that she is several inches taller than Danzer.