Black-Tailed Gull is a type of Gull Friend that was first revealed on Twitter.
Black-Tailed Gull seems to be a shy and gentle friend: a loose stature, half-closed dandelion eyes, and a small smile. Her long hair is primarily snow white, but in the back fades to a blue-grey, then to black. On her bangs is the characteristic avian hair strand, decorated in pastel yellow, crimson, and charcoal. Her head-wings and fan tail are made up of the same palette. As for her clothes, she dons a white sailor dress with a lavender ribbon, navy sleeves, and a denim ruffled end. Around her wrist she wears what appears to be a red collar as a bracelet, which could be an allusion to a tracking tag. On her legs are a pair of yellow tights, and loafers of a slightly darker shade.
In Real Life
The Black-Tailed Gull is a medium avian native to East Asia, most well known for it's call and indicative black tail its named after. The bird has cat-like vocalizations, resulting in the naming of the animal in East Asian languages naming them as a variety of feline-based titles, such as "Umineko" (Sea Cat) in Japanese, or "Gwaeng-Yi" (Cat Gull) in Korean. Like other gulls, they naturally consume fish, mollusks, and crustaceans, but human interference has caused them to sometimes consume leftover scraps or carrion.
In Shinto mythology, the Umineko was seen as a messenger to the goddess of fishing, and protected/fed by the local Japanese population. A shrine was built to honor the creature, and ever since, 40,000 gulls flock to the shrine site every summer to nest. These gulls are a popular tourist attraction, the shrine being a National Monument of Japan.