Flat Escardos hails from the Mediterranean-based Escardos lineage of Magi. As the eldest son, his exceptional Magic Circuits and innate talent initially marked him as a prodigious successor. During childhood, he displayed unprecedented abilities by deciphering ancient family Magecraft texts incomprehensible even to the family head and innovatively combining historical spells into new forms. However, this perceived abnormality prompted his family to repeatedly attempt his assassination. These attempts failed because Flat could perceive an unnamed presence ("it") invisible to others, warning him of danger. After the fifth failure, his parents sent him to the Clock Tower under the guise of education to remove him from the family.

At the Clock Tower, Flat faced rejection from multiple departments despite his unparalleled magical aptitude. Professors struggled with his unconventional mindset and absence of typical Magus traits like secrecy or solemnity. His habit of altering professors' magical formulas mid-lecture and disrupting classes with eccentric behavior nearly caused his expulsion. After a year of instability, Rocco Belfeban, head of the Summoning Department, referred him to Lord El-Melloi II's Modern Magecraft Theory class. During their first meeting, El-Melloi II instantly detected Flat using Magecraft to force a smile, criticizing his focus on results over process. This honesty elicited Flat's first genuine smile, and he joined the class as a corner-seated student.

Flat's personality merges genius-level Magecraft proficiency with a startling lack of common sense. Described as whimsical and childlike, he exhibits a "superabundance of curiosity" rather than strategic or logical thinking. His interests lean fantastical, like his enthusiastic desire to witness the Holy Grail due to its association with figures such as Hitler, Qin Shi Huang, and Godzilla. This dichotomy labels him both a top student and a notorious problem child. Despite his intelligence, behavioral issues stalled his academic progress; he remained one of El-Melloi II's oldest students at nearly twenty years old, though his raw talent prevented expulsion.

His magical abilities transcend conventional limits. He manipulates Magecraft irrespective of Od, Mana, or formal knowledge, effortlessly modifying spells and creating new ones. Notably, he deciphered the Voynich Manuscript, inadvertently causing extradimensional plants to invade a botany class—resolved with unwilling assistance from Ayaka Sajyou. His perception of "it" remains undefined but grants unique survival instincts. Later, during the Snowfield Holy Grail War, he summoned False Berserker (Jack the Ripper) using a toy knife catalyst. He teleported False Berserker to safety during True Archer's assault and manipulated True Archer's magical energy via Command Spells, despite not being the latter's Master.

Interpersonally, Flat is universally regarded as affectionate and carefree, making him difficult to dislike. He shares a contentious yet profound camaraderie with classmate Svin Glascheit, whom he nicknames "Le Chien" (The Dog) due to Svin's Beast Magecraft. Svin initially resents the nickname and perceives Flat as dangerously unstable but eventually forms a legendary academic rivalry. Flat's influence extends beyond class; his infiltration of a secret meeting about the False Holy Grail War underscores his reckless curiosity. Furthermore, his existence ties into the Escardos family occult legacy: upon Flat's death, the entity Thia manifests within his body as a secondary self, contingent on specific familial criteria related to their Magic Crest inheritance.