Therese Alexanderite is a member of the Jewel race from Glass' World, recognized by her light blue hair and eyes that shift color during spellcasting, ivory-like skin, and a three-strand braid. She wears gem-embedded bangles on both wrists. Typically serious and deferential, especially in contrast to her companion L'Arc Berg, she exhibits intense passion for gem craftsmanship. This passion escalates into a boisterous demeanor when using powerful accessories, earning her the nickname "Motoyasu III." She communicates with gemstones, deeply concerned for their well-being, and draws her magic power from them. The quality and refinement of her accessories directly amplify her magical capabilities; three accessories crafted by Naofumi temporarily boosted her power to rival a Legendary Hero. Her Jewel Magic includes spells like "Shining Stone: Ball of Flame" for selective enemy purification, "Shining Stone: Showering Thunder" for lightning strikes, "Shining Stone: Shooting Star Rain of Fire" to summon meteors, and "Shining Stones: Hardened Protection" for defense. She also uses illusion magic for party disguises and combination skills: "Thunder Rain Flying Circle" with L'Arc merges scythe techniques and thunder magic, while "Reverse Burning Snow Moon Flower" with Glass combines flame spells and wind attacks. Other abilities encompass paralysis-inducing butterflies, sleep-inducing smoke, and ocean vortex nullification. Initially traveling with L'Arc and Glass to Raphtalia's World to confront the Legendary Heroes, her level reset upon arrival. At the Cal Mira Islands, she befriended Naofumi's party, commissioned a bracelet from him, and fought alongside them against Waves until recognizing Naofumi as the Shield Hero and retreating. Later, she rejoined the conflict against Kyo Ethnina at the Spirit Tortoise, temporarily allying with Naofumi to halt Kyo's mass murder before pursuing him through a dimensional rift. This alliance solidified after reconciliation facilitated by Kizuna Kazayama. Subsequently, her obsession with Naofumi's accessory craftsmanship grew, leading her to exchange jewel magic lessons for observation rights. During assaults on Kyo's laboratory and vanguard Waves, she sacrificed a Naofumi-made accessory to reclaim L'Arc's stolen Vassal Scythe, receiving a loaned replacement that further amplified her power. She strategically leveraged this accessory-driven fervor, using Naofumi-crafted items as bait to persuade enemy jewel users to defect during battles.