The Gionne family is well known and respected throughout Europe for their successful export-import business. Excella's model-like beauty and aristocratic background makes her haughty towards those around her, especially men. Gifted with a keen intellect and inheriting her father's business acumen, Excella quickly breezed through school and enrolled in a university at a startling young age, where she majored in genetic engineering. Her grandmother is of the Travis family, the founders of TRICELL, which has bestowed Excella with quite a noble and storied lineage.
Around 2005, around the time of the Queen Zenobia Incident, Excella briefed Jessica on those involved in the mission, as well as their and her histories. She eventually accepted Jessica's request, although under the condition that she send in some "homework."
"As soon as you two are done with your little 'vigilante mission', you should leave. There's nothing here worth throwing your lives away for..."
— Excella to Chris and Sheva
After revealing the location of the facility through his possession of Oswell E. Spencer's personal records, Wesker demonstrated the effective usefulness of Plaga by developing the Type 2 Plagas and Type 3 Plagas with assistance from TRICELL researcher and employee of Excella, Ricardo Irving, who most likely provided the pair with Plaga samples off the black market. This allowed Excella and Wesker to produce their own army of Majini as soldiers to help fend off persistent BSAA agents, Chris Redfield and Sheva Alomar. The two had been dispatched into the area to investigate TRICELL's mysterious project and later refused the order to retreat since Chris was in search of his previously thought dead partner, Jill Valentine. Wesker also entrusted Excella with his special vaccine, PG67A/W, which she administered to Wesker daily to keep his viral superhuman powers in balance and kept in attache briefcases beside her at all times.
Chris and Sheva found out about Excella's involvement in the odd events transpiring from the defeated Irving, who uttered her name to them in his dying breaths. They eventually made it to the facility and Sheva told Chris that she had heard the name before, and that it was the name of the CEO of TRICELL Africa. Much deeper in the underground center, they discovered that TRICELL had been kidnapping thousands of human test subjects and keeping them in cryogenic capsules. They found Jill in the computer and the platform they were on started going down to reach her pod. However the platform stopped before reaching the pod, blocked by the B.O.W.U-8 which the agents were forced to fight. When reaching Jill's pod, the agents discovered it was empty and then were contacted by Excella over the computer screen. Sheva asked why she was doing the strange experiments in the facility. Chris also asked where Jill was, though Excella denied knowing anything, saying she didn't know about Jill, and warned them to leave on the risk of their lives. Sheva could tell that Excella was lying and Chris guessed that Excella was the one who sent U-8 to kill them. The two agents pressed on in search of Excella.
"I believe I've proven I'm worthy, haven't I?"
— Excella to Wesker
In a control room much deeper in the facility, Excella was shown injecting the PG67A/W serum into Wesker himself and informing him that they could soon leave, as preparations for their ship were almost complete. Excella expressed her initial doubt and surprise upon Wesker's arrival that Plaga and Uroboros had been so successful. This secured her position within TRICELL, but Excella mentioned that she intended to uphold a much more significant authority, asking if she had proven herself worthy of being Wesker's partner in his new world, and making some sexual advances on him. Wesker said she may have proven herself but pushed her away after grabbing her face in a threatening manner, rejecting her advances due to being such a cold villain. The cloaked female working for the two came in and informed them of Chris and Sheva's arrival into the facility. Excella noticed that Wesker seemed concerned about Chris, but he said he would not tolerate delays, and she left along with the cloaked woman and a briefcase full of PG67A/W.
Chris and Sheva soon heard over radio transmissions Excella and Wesker talking, leaving Chris distraught that his old nemesis was there. They soon made it to a strange experimental lab and found a silent test subject in some sort of a mechanical chair. Excella called out to the pair over an intercom from an observation window above the room with the cloaked woman beside her. Chris again demanded to know where Jill was, but Excella shrugged off the statement with "Jill, Jill, Jill. You're like a broken record, you know that?" She talked about evolution as the subject began to rise and transform, confusing Chris and Sheva. It eventually turned into the horrific monster Uroboros Mkono and Excella and the woman left the agents to fight the creature.
After defeating Mkono, the pair found Excella in a room deeper in the center, demanding again to know where Jill was. She refused and the pair were attacked by the masked woman. Her mask was knocked off in the battle and Wesker soon entered. He revealed that the woman was in fact Jill, who was under influence of P30 chemical. She and Wesker proceeded to battle the two, and Excella vanished. During the fight, Wesker received a call from Excella and left Jill to fight the agents alone. Excella and Wesker boarded their large tanker with Chris and Sheva, having returned Jill to normal, in pursuit. They soon found Excella in a room on the boat filled with Progenitor flower specimens packing two cases with PG67A/W vials. Held at gunpoint, she quickly closed the brief cases while the two tried to interrogate her again. She casually denied to say any information and found an opening to run for a door. Chris and Sheva fired to try and stop her, but only managed to knock one of the suit cases out of her hand. She still managed to escape. Sheva commented that she was a bit tough for the move she just made and picked up the vials she dropped. Chris and Sheva kept them, knowing they must have been important for Excella to be so protective of them.
"Albert... You said we'd change this world together! Why!?"
— Excella to Wesker moments before her transformation
Unfortunately for her, Excella realized the foolishness of trusting Wesker when it was too late, and she paid the ultimate price for it. As Chris and Sheva came across an open area of the deck of the freighter, Excella was seen, next to an enormous pile of Majini corpses, in the throes of agony. The pair aimed their guns at her and Chris demanded to know what was going on. Excella cried out to Wesker in pain, asking how he could betray her after everything she'd done for him. From afar, Wesker spoke through a PA system, taunting her as Chris and Sheva looked on. He gloated about his new world to come, revealing Excella was just a pawn to him, and that he had injected her with Uroboros. Excella dropped to her knees and threw her head back as Wesker hinted that he knew all along that she would not be compatible with Uroboros, and said that he had one last task for her.
After one last heartbroken cry of "Albert!", Uroboros Aheri erupted from Excella's mouth, transformed her and mutated her body completely, absorbing the dead, and growing to massive size before battling Chris and Sheva. The two agents had to run for the inner workings of the freighter to avoid being killed by the monstrosity. It appeared that the creature retained its intelligence, as it reached through certain parts of the ship to grab Chris and Sheva. Wesker also managed to avoid the grasp of the beast by going below the deck and into a hanger deep within the ship, ready to go through with the plan without Excella. Chris and Sheva headed to the roof as the beast began to tear through the ship. Chris and Sheva made use of a satellite laser on the ship's roof, destroying the monster's primary weak points and freezing it into nothing more than a giant mass of condensed tentacles.
Throughout the course of the game, some romantic affection can be seen through Wesker and Excella. She also considers herself a "Queen" in, what would have been, Wesker's New World. However, Wesker only used Excella's romantic interest to further his plans with Uroboros and Chris told Sheva that "Wesker doesn't give a damn about anyone but himself".