Amelia Wil Tesla Sailoon is the second princess of the kingdom of Sailoon, a realm renowned as a center for white magic. She is the younger daughter of Crown Prince Philionel el de Sailoon, and her mother was murdered by an assassin when Amelia was very young. Following this tragedy, her older sister, Gracia Ul Naga Sailoon, left the kingdom, leading to rumors, later confirmed by the series creator, that Gracia is the character known as Naga the Serpent. Amelia was raised primarily by her father, and her other relatives include her grandfather, the bedridden King Eldoran, as well as several uncles and a cousin who were involved in failed plots to usurp the throne.

Amelia is a self-proclaimed champion of justice, a passion she inherited from her father. She is utterly devoted to the cause of truth, honor, and dignity, and she pursues her quest with an energetic, single-minded, and often naive enthusiasm. Her view of the world is initially very simplistic, dividing people and situations into clear categories of good and evil based on superficial stereotypes, such as assuming all villains wear black. Despite this idealism, she can be a klutz, has a reputation for being one of the clumsiest characters, and often looks for the highest point possible to make a dramatic, justice-themed speech, only to fall from it. Although she is a princess, she rarely uses her royal status, only doing so on specific occasions to help others. She also has a notable physical trait: she is younger than her friend Lina Inverse but is more physically developed, which is a source of frustration for Lina.

Amelia's primary motivation is the active pursuit of justice, which serves as a personal mission to right wrongs and protect people. Her deep-seated need to believe in the triumph of good is partly a coping mechanism developed in response to the traumatic loss of her mother during her childhood. This idealism drives her actions and her unwavering commitment to helping those in need, even if her methods are sometimes misguided. She also seeks to become a hero, initially idolizing Lina Inverse as a model for this goal, though she eventually strives to define her own path.

Within the story, Amelia functions as a core member of the main party of adventurers, acting as the group's moral compass and providing both healing and offensive magical support. She joins Lina and Gourry while searching for her father and continues to travel with them, contributing to their confrontations with a wide array of powerful enemies, including the Copy Rezo, the Mazoku Lords Gaav and Phibrizzo, and the dark entity Dark Star. Her royal and diplomatic background also proves useful during their various political entanglements.

Amelia's most significant relationship is with Lina Inverse. She initially views Lina with hero-worship, wanting to emulate her, but this evolves into a solid, sibling-like friendship where they support and tease each other. Her dynamic with Zelgadiss Graywords is also central. While she first finds him creepy, she grows to respect his good heart, and the anime strongly hints at her having romantic feelings for him, with the two often providing grounding and support for one another. She treats the swordsman Gourry Gabriev like an older brother, and though they are not extremely close, they share a comfortable camaraderie. Despite knowing he is a Mazoku, she remains polite to Xelloss and even uses her optimistic speeches to tease him. She also develops a friendly rivalry with fellow princess Martina Zoana Mel Navratilova and a respectful relationship with the golden dragon Filia Ul Copt.

Over the course of her adventures, Amelia undergoes significant character development. She begins as a very naive and judgmental idealist who cannot distinguish between good and evil without relying on cliches. Through her experiences with Lina and the group, she learns that the world is not simply black and white, and her perspective matures considerably. She becomes less quick to judge and more understanding of moral complexities, while still holding onto her core belief in justice. Her skills as a mage and a fighter also improve, and she grows from a somewhat sheltered princess into a more confident and capable individual, even commanding a battalion of magic users in Slayers TRY.

Amelia is a highly capable magic user specializing in white magic, which she uses for healing and support, but she also has extensive training in Shamanistic magic, particularly astral shamanism, where her skill is noted to be greater than Lina's. She can also wield some black magic spells. Her signature spell is Ra Tilt, which she considers her killer spell. She has even developed an original spell, Visfarank, which manifests magical energy as glowing boxing gloves around her fists. In addition to her magical prowess, Amelia prefers hand-to-hand combat and possesses immense physical strength and durability, far beyond what her small stature would suggest. She is capable of punching through magical barriers, crushing boulders, and surviving massive explosions and falls. This combination of powerful magic and formidable physical ability makes her a surprisingly effective and resilient fighter on the front lines.