Locusta is a Assassin-class servant from Fate Grand Order. A notorious poison maker of Ancient Rome and a favorite of the Emperor Nero. She is said to have supposedly taken part in the assassinations of Claudius and Britannicus, and taught Nero's other poisoners. After Nero's death, she was executed along with Nero's other favorites by her successor, Galba. Locusta was discovered by Nero's mother, Agrippina, and is said to have created the poison that she used to kill Emperor Claudius. Later she also participated in Nero's assassination of her stepbrother Britannicus.

Her appearance is young having cyan hair with purple ends, she also has purple eyes. She often wears a long purple ceremonial shoulder cloak concealing a pinkish swimsuit looking top and bottom. She also wears a hat shaped like a mushroom cap but is is a matching purple and pink color.

Her skills and even form of combat involve making use of mushrooms of all kinds to brew medicines, foods and especially poisons. Locusta's first Noble Phantasms is Finis Fungus. Forcing a simmered mushroom stew down the target's throat. More precisely, throwing the liquid at them. Needless to say, the stew is toxic. Locusta's second Noble Phantasm is Finis Venenum Nero. The most refined poison, which she produced only once in her life, poured her whole soul into, and stored in a small golden box. It grants the target a peaceful slumber, quietly extinguishing the fire of their life.