Sebaceous cyst: A common, benign cyst that can occur anywhere on the skin, including under the helix. The fluctuant nature suggests a fluid-filled cyst, which is consistent with a sebaceous cyst.

Epidermoid cyst: Similar to a sebaceous cyst but lined by epidermal cells, these cysts can also present as fluctuant masses under the skin.
Lipoma: A benign tumor composed of fat tissue, which can feel fluctuant and is commonly found just under the skin.
Abscess: Although the patient has no history of infection, an abscess could still be considered, especially if the mass is tender or if there are other signs of infection.

Malignant tumor (e.g., basal cell carcinoma, squamous cell carcinoma): While less likely, it's crucial to consider malignant tumors, as they can have significant implications if not diagnosed and treated promptly.
Infected cyst or abscess with atypical bacteria: Certain infections, such as those caused by MRSA, can present without a typical infectious history and can be serious if not properly treated.

Branchial cleft cyst: A congenital anomaly that can present as a fluctuant mass in the neck or near the ear, though it's less likely to be found just under the helix.
Dermatofibrosarcoma protuberans: A rare skin cancer that can present as a firm to fluctuant nodule, though it's uncommon and typically found on the trunk or proximal extremities.
Pilomatrixoma: A rare, benign tumor of the skin that can present as a firm or fluctuant mass, often in children or young adults.