Vespasien, titled "The High-Pressure Artificer," is a 5-star Pyro claymore user from Fontaine, associated with The Fleuve Cendre Under-docks. Physically tall and robust, he has messy, auburn hair and glowing amber eyes. His unique industrial-Victorian attire includes a fitted tailcoat, heavy gauntlets, and a distinct foundry helmet with a miniature smokestack. Personality-wise, he is boisterous and obsessed with mechanics, enjoying the thrill of steam explosions and modifying devices. His combat skills focus on powerful, Pyro-infused attacks, utilizing heavy swings and explosive steam abilities to deal damage and inflict status effects. His elemental burst, "Catastrophic Overload," unleashes devastating AoE damage while regenerating health based on his pressure stacks.

Vespasien is a character with several unique abilities and ascensions in gameplay. His ATK Speed increases by 8% and Normal Attack DMG by 10% with up to 3 stacks. His fourth ascension passive, "Venting System," generates stacks of "Boiler Pressure" when he takes damage and cleanses elemental debuffs during his Elemental Burst. Additionally, his utility passive, "Forge-Furnace," offers a chance to create enhanced dishes and reduces crafting times for weapons. Vespasien's constellations enhance his stats and abilities, such as increasing HP, modifying damage resistance, and adjusting attack mechanics based on "Boiler Pressure" stacks. He has an extensive list of required materials for ascension and talent upgrades, including Mora and various gem types. Vespasien's lore reveals his background, struggles, and ambitions, including his desire to improve heating systems for those in need. His Pyro Vision symbolizes his fiery passion for survival, spawned during a dire winter. His character underscores themes of resilience and innovation amidst adversity.

Vespasien is characterized by his enthusiasm for thermodynamics, often expressing his views through a series of humorous and exaggerated remarks about steam power, humidity, and machinery. He reacts to varying weather conditions with confidence, emphasizing his forge's resilience and the importance of steam as a force of ambition. His interactions reveal a playful yet intense relationship with characters like the Duke of Meropide, journalist Charlotte, and Captain Chevreuse, showcasing his creativity and belief in steam's transformative potential. As he undergoes phases of "ascension," he embodies a lively and dramatic blend of engineering fervor, merging human and machine in a whimsical yet powerful display of personality.