In the American fantasy novel "Which Immortals Live in the Mansion?" by H.P. Lovecraft, the mansion serves as a home to a variety of supernatural beings and immortals. The most prominent immortals that reside within the mansion include the Outer Gods, the Elder Gods, and the Ancient Ones. These entities are ancient and powerful beings that transcend human understanding and are often associated with cosmic and existential themes.

The Outer Gods are the most immediate residents of the mansion, as they are the primary focus of the novel. They are beings of immense power and influence, often depicted as beings that are beyond the scope of human comprehension. The Elder Gods, while less frequently mentioned, are also significant immortals that are believed to be the creators of the universe and the Outer Gods themselves. The Ancient Ones, on the other hand, are even more ancient and mysterious, and their presence in the mansion suggests a deep connection to the origins of existence.

The reason these immortals are associated with the mansion is due to the thematic elements of the novel. Lovecraft's work is known for its exploration of the unknown and the terrifying, and the mansion serves as a physical manifestation of these themes. The mansion is a place where the boundaries between the human world and the cosmic unknown are blurred, allowing for the manifestation of these powerful beings. The data supporting this analysis comes from the novel itself, which provides detailed descriptions of the mansion and the immortals that inhabit it.

For instance, the novel describes the mansion as an ancient and decaying structure that seems to defy the laws of nature. This setting is indicative of the immortals' enduring presence, as they are beings that have outlived the ages and continue to exist in a realm that is separate from the human world. The Outer Gods, in particular, are often described as beings that are both benevolent and malevolent, and their presence in the mansion suggests a complex relationship with humanity.

Furthermore, the novel's narrative structure and the use of first-person perspective contribute to the immersive experience of encountering these immortals. The protagonist's journey through the mansion and their interactions with the immortals are central to the story, and this first-person perspective allows readers to experience the novel as if they themselves were exploring the mansion and encountering the immortals.

In conclusion, the immortals that live in the mansion in "Which Immortals Live in the Mansion?" are the Outer Gods, the Elder Gods, and the Ancient Ones. These beings are central to the novel's exploration of the unknown and the terrifying, and their presence in the mansion serves as a physical manifestation of the novel's thematic elements. The novel's narrative structure and first-person perspective further enhance the immersive experience of encountering these powerful beings.