Once a mischievous trickster that delighted in confusing Highland travelers, the Shellycoat was bound to service by Angus MacTavish's ancestors during a ritual to strengthen the protective barriers around Loch Ness centuries ago. Over generations, the creature developed a genuine loyalty to the MacTavish clan after witnessing their dedication to maintaining the balance between worlds, eventually becoming a willing guardian that uses its deceptive abilities to protect rather than perplex.

The Shellycoat Sentry possesses remarkable sound manipulation abilities, capable of creating misleading echoes, whispers, and calls that lure intruders away from sensitive areas while also detecting disturbances in the supernatural balance from miles away. Its body can partially dissolve into the waters of Loch Ness, allowing it to move with incredible speed through connected waterways and reappear wherever its protection is needed. The ancient shells adorning its form are inscribed with Celtic protective runes that resonate with harmonic frequencies when danger approaches, serving as both warning system and magical barrier against malevolent energies attempting to penetrate the loch's defenses.

The Shellycoat Sentry patrols the boundaries between safe and forbidden zones around Loch Ness, using its auditory illusions to confuse and redirect those who venture too close to places where the veil between worlds grows thin, while simultaneously communicating vital intelligence to Angus MacTavish through a system of coded sounds that only trained Guardians can interpret.