Scream is a symbiote spawn of Venom and an enemy of Spider-Man.
The Life Foundation
Little has been revealed about Diego's early life. Donna had a history of psychotic episodes, but despite this she passed a background check and became a security guard for the Life Foundation. As part of their program to create a super-powered security force, the Life Foundation scientists kidnapped Venom. They removed his symbiote's "seeds," hatched them and merged the resulting creatures with five of their most skilled and trusted employees. One of those employees was Donna.
While testing her new powers, Diego attacked a mall in Salinas, California. She encountered Spider-Man, who was looking for Eddie Brock and fled back to the Life Foundation's base in the Mojave Desert. Spider-Man stowed away on her hovercraft and released Brock. Diego and the other new symbiotes lost the resulting battle, and they apparently died from accelerated aging when Brock used a metabolic accelerator on them. Her symbiote was turned into dust and her body caught in a massive explosion.
It was later revealed that Scream and the other symbiotes had survived due to efforts by the Life Foundation. Shortly afterward though, Donna left with Lasher, Phage, Agony and Riot to find Eddie Brock in New York. They had trouble coexisting with their immature symbiotes and hoped that Brock would work with them so as to teach them control. Brock refused and then disappeared. Diego asked the Scarlet Spider for help in finding Brock. When he also refused, Diego had a psychotic episode and went wild in Times Square. She succeeded in drawing the attention of Brock and Reilly. Brock fought with Diego, and only the intervention of Reilly saved her.
The other "guardian" symbiotes joined Diego in her next attempt to get Brock's help. They freed Brock, who had lost his symbiote and gone to jail, and took him to Chicago. He managed to escape. Diego, whose schizophrenia gave her symbiote a powerful psychic weapon, killed Gesneria, Mach, Cole, and Hernandez one by one and blamed Brock for the murders. Brock reunited with Venom before Diego could kill him as well and defeated her.
Reform & the Xenophage
Diego somehow gained a measure of control over her symbiote, or at least reached a modus vivendi with it. To make up for her past behavior, she began seeking out other symbiotes and helping their human hosts to cope. She soon learned of an alien called Xenophage that preyed on symbiotes. With Brock's help she defeated it.
In Diego's most recent appearance, she found the Xenophage's ship. Along with Brock and Wolverine, she traveled to an alien dimension and returned safely to Earth.
When Eddie Brock started his hunt for symbiotes, he killed Donna after Hybrid. Brock lead Scream to a trap, where using a sonic emiter, paralized her, and stabbed her in the head many times with a heated dagger.
Scream was one of the five Life Foundation Symbiotes that were forcefully spawned from the Venom symbiote. Unlike her "brothers", she wasn't merged into the Hybrid symbiote, being still at large.