Thalassia is a slender sea elf with blue-green skin, long silver hair that moves as though suspended in water, and luminous turquoise eyes. Fine, iridescent markings curve from her temples down her neck like the patterns of deep-sea creatures. She wears weathered leather armor beneath a dark scholar's coat, decorated with shells, glass specimen vials, and fragments of coral. A faint shadowy tentacle often curls protectively around her wrist when her patron's power stirs.

Quietly observant and intensely curious, Thalassia treats every creature and environment as a source of knowledge. She is patient with honest students and surprisingly compassionate toward frightened people, but becomes cold and relentless when scholars, sailors, or powerful institutions treat the natural world as something to be conquered.

Thalassia was raised among the coral libraries of a sea elf enclave, where ancient charts were etched onto shell, whale bone, and translucent membranes harvested from the deep. Her natural curiosity drew her beyond the familiar reefs and into lightless trenches where even experienced explorers feared to descend. She served as a researcher and expedition scholar, recording strange currents, bioluminescent ecosystems, and ruins older than the oldest elven songs. Her greatest discovery was a sunken observatory whose walls pulsed with a living blue light and whose sealed chamber whispered her name in a voice like distant surf. Inside that chamber, Thalassia encountered an immense presence known only through grasping shadows and pressure that bent the water around her. The entity offered knowledge of the abyss in exchange for a promise to protect the hidden places of the sea from those who would exploit them. Thalassia accepted, believing that understanding the deep was worth any personal danger. She emerged with eldritch power, a spectral tentacle at her command, and an unsettling certainty that something vast now watches the surface through her eyes. She travels between coastal settlements and inland institutions, seeking lost knowledge, rescuing endangered creatures, and investigating signs that the abyssal presence may be preparing for a change in the tides.