In circa 1991, Hilda contracted an incurable and fatal disease indigenous to the region of South America in which she and her family lived. Years later, Javier was contacted by an Umbrella researcher, offering him the usage of the t-Virus to cure Hilda of her illness. After the t-Virus was administrated, her health had improved and destroyed the cancer cells within her body, two months later, she had regained much of her strength. It didn't take long for the t-Virus to backfire, causing Hilda's health to rapidly decline, transforming her into a monstrous, amphibian creature unlike any naturally-occurring organism. Mostly, Hilda is a mindless beast, but she seems to retain enough humanity to recognize her daughter and will not attack her. Indeed, the sound of Manuela singing the lullaby that Hilda used to sing to her causes the beast to become docile for as long as Manuela can keep it up.
In 2002, Hilda somehow escaped Javier's mansion and made her way down river and into Mixcoatl's waters during the village's t-Virus outbreak, she was briefly seen by Leon S. Kennedy and Jack Krauser after they abandoned their original guide for Amparo. She appears to pull the guide's corpse through a hole in the floor, an event that does not go unnoticed by the pair. After retreating to the Mixcoatl Church, they notice the creature calmly observing her daughter Manuela. Manuela falls unconscious for whatever reason and Hilda rises up to fight Leon and Krauser. After retreating outside, they open fire on the flimsy bell holder on the church room to make it fall on top of her, causing her to retreat. She later appeared out of nowhere above the operating room inside Javier's Mansion, allowing Javier to escape to the green house as she engaged Leon and Krauser. Following a long battle, she fired bone spears in Krauser's direction, impaling him in the arm. They both proceed to shoot her repeatedly until she falls to the ground. As she lay dying, she bids her daughter farewell, shedding a tear and finally succumbs to her wounds.
In her mutated state, Hilda possesses long, powerful tentacles that serve as gripping arms, the ability to spit acidic or toxic fluids, tremendous strength, and the ability to launch deadly bone projectiles from her tail. Her new biology is primarily suited for an aquatic environment, but she is fully capable of fighting on land as well.
In Chapter 1, Hilda's entire body is vulnerable to damage. By the end of Chapter 5, she has hardened her skin to become bulletproof, also turning most of her body red-orange in the process. Fortunately, the development of her carapace has also destroyed the black plating on her face and back, exposing her heavily deformed face and a segment of surprisingly human-looking brain matter near the base of her neck. Leon and Krauser must target these spots to bring Hilda down once and for all.
- It is possible that the t-Virus used as an attempt to cure Hilda was the T-JCCC 203, an experimental t-Virus variant that was created by Umbrella meant to cure cancer cells, which backfires on its users.
- It was one of Hilda's bone spikes that crippled Jack Krauser's left arm and caused him to be let go from SOCOM, eventually joining Wesker.
- Hilda's eye color changes depending on her behavior, blue when she is in her calm and gentle state, red in her hostile state.