Alyssa first appears in the Fuuka Academy choir, where she is well known around the school for her wonderful singing voice and is commonly called the "Golden Angel". Mikoto Minagi and Mai Tokiha encounter her on a hillside singing with Miyu Greer during a day off, where they exchange a few words. Alyssa is frequently seen clinging to Miyu, and rarely speaks unless spoken to, coming off as very shy and timid. She is in the Fuuka elementary school.
Later, she is seen summoning an Orphan with Joseph Greer by her side, which causes her to collapse from the strain of it. She is formally introduced as an antagonist monitoring the HiME in episode 12 after Natsuki discovers them to be using the basement of the church as an observation center. The Searrs foundation that she is a part of is noted for influencing many world events (such as the United States presidency) and are responsible for the recent "vampires" plaguing the school, which are actually Orphans taking the blood of female students to find the HiME. Searrs' Board of Directors, with Alyssa's adoptive father as the CEO, plan on seizing the power of the legend of the "Battle Princesses" to usher in their own version of a "Golden Age".
Alyssa was part of a genetic engineering program to produce a "fake" HiME, of which she was the only success (the tag on her foot in a flashback lists her as "Valkyrie No. 143"). Miyu was a super intelligent android created by Joseph Greer to be her guardian, as she was unable to summon an element since she was not a true HiME. However, Alyssa's glowing hair briefly allows her to distract the HiME pursuing her and can seemingly be used to teleport herself and Miyu away from danger. Her hair glows whenever she summons an Orphan or uses any of her "fake" HiME powers, including calling her Child.
After the location of the observation is discovered and the vampiric Orphan is defeated an angry Joseph Greer is dismissed by Alyssa's father (with Miyu acting as a communication device) and he leaves only after realizing the secret codes he installed in Miyu have all been removed.
When the Searrs foundation begins its invasion of Fuuka Academy Alyssa and Miyu attempt to capture Mashiro, only to find that the headmaster had used a decoy and already escaped. In a final confrontation her Child is destroyed by Kagutsuchi and Mai after they leave the atmosphere to protect the other HiME, and Alyssa collapses as her most important person (revealed to be her father) disappears at the Searrs headquarters, where the remaining members comment that they have to wait another 300 years to try again. Miyu carries away the weakened Alyssa, but she is found by Joseph Greer, who shoots Alyssa under orders of the Searrs corporation for her failure. A distraught Miyu cries that the golden light she loved has disappeared and carries Alyssa's body into a frozen lake, where it is assumed Miyu herself perished.
In episode 23, it is revealed that Mashiro recovered Miyu (but not Alyssa) and a great deal of Joseph Greer's Searrs technology to repair, and even reformat, Miyu. Midori wakes up Miyu, who then escapes and later confronts Nagi in the library to gain access to the hidden underground cavern of the HiME.
A small presence in the form of a flame that spoke with Alyssa's voice activated Miyu's "Platinum Mode" and anti-Materialiser weapon during the final battle. In the end Alyssa is revived by Mashiro with the other HiME. It is assumed her father was also resurrected with the other Most Important People of the HiME, though Alyssa's specific resurrection is not shown on screen.
Her child, Artemis, takes the form of a huge satellite orbiting the Earth that can fire a massive energy beam capable of destroying bridges, as well as launching a swarm of missiles for point defense. This was Searrs' plan to wipe out the other HiME, leaving Alyssa as the Festival's winner; then Artemis would destroy the Black Prince as well, leaving Searrs with a fully functional HiME and Child for their Golden Age.
The song Alyssa is seen singing is titled "It's Only The Fairy Tale", which is an "Engrish" rendition apparently about the HiMEs, as the lyric "Who are those little girls" and "Twelve of them" refer to the fact that there are 12 HiMEs; all of which are female.