Dahlia wears a ceremonial dress and headdress with a red and black striped tie, and always seems to be barefoot. According to the European game manual, Dahlia is the proprietor of the Green Lion Antiques store in Silent Hill. It is also rumored that she has a side business of magic spells and fortune-telling to her customers.
Alessa's presence has summoned poltergeist activity since she was a child. These disturbances, along with Alessa's latent telekinetic abilities, branded her as a witch by her classmates. Dahlia knew of her daughter's powers, and surmised they were linked to the dark "God" which the Order worshiped. Hoping to "birth" her God through Alessa, Dahlia used occult-like rituals to impregnate her. One of these rituals resulted in a house fire, with Alessa trapped inside. Although the fire was ruled an accident, Alessa was left completely disfigured from the resultant burns.
Through the use of a magical incantation invoked by her mother, Alessa lingered despite her fatal injuries and was placed in the care of Dr. Michael Kaufmann, a cult affiliate. While the constant torture of her wounds helped refine Alessa's reality-bending powers, she had resisted being used as a container for the 'God', and her soul split in two halves, using a form of bilocation in order to escape her torture. Her other form, a newborn, was discovered by Harry Mason and his wife, who named her Cheryl and raised her as their own.
At some point over the course of her captivity, Alessa overheard a conversation about a "special visitor" she was to receive, and became aware of her mother's intent to still carry out her plan to use her to "birth" the God. Alessa realized that for however long she continued to live, this remained a possibility. Knowing that Cheryl held the other half of her power, Alessa summoned her telepathically back to Silent Hill so she could end her life and prevent the God's birth.
During this time, Kaufmann grew impatient after waiting for the return he had been promised (wealth, presumably) for his role in the conspiracy, leading to a falling out with Dahlia. Kaufmann stashed vials of Aglaophotis, a chemical capable of "aborting" the demon in Alessa and ending Dahlia's plans, in case she ever turned on him.
Beyond her involvement in Silent Hill and Silent Hill 3, Dahlia has also been given connection to the serial killer Walter Sullivan. When he was just a young boy, he was told by Dahlia that his mother was "asleep" in Ashfield. Throughout the diary entries in the Forest World, Walter wrote of an important lady named Dahlia. Further connection of Dahlia and Walter can be found in the Wish House Orphanage just before the death of Jasper Gein, where Henry Townshend can find a note asking if Alessa has been found and also requesting a report of Walter's progress that is supposedly involved with the 21 Sacraments and the hastening of God's arrival.