Hermit Purple manifests itself as multiple, purple, thorn-covered vines that spawn from Joseph's hands. The vines are strong enough to support his own weight and long enough to bind someone completely.
Hermit Purple is consistently portrayed as being purple across all media featuring it, befitting its name.
Hermit Purple represents the Tarot Card The Hermit, which symbolizes introspection and contemplation, best represented in the Stand's powers of divination.
The series is known for alternating colors between media, the information presented below may or may not be canon.
(Purple and blue)
Hermit Purple's vines are relatively weak in combat, but they provide Joseph with considerable divination ability, enabling him to materialize any type of information, such as an image, a location or even thoughts.
Joseph demonstrated that he can use the vines as ropes, whose movements he can control at will. Their length and resistance make it possible for Joseph to swing from place to place and tie someone. The vines have limited resistance as a sufficiently strong Stand or even a strong Vampire can break them. The vines are dexterous enough to manipulate a video game controller.
Hermit Purple divining information from a television
Hermit Purple's main ability is to divine information. By using any object or material in conjunction with Hermit Purple, Joseph Joestar can display any information he wishes to see. To date, he has used Hermit Purple to:
Produce psychic photographs by destroying a camera. The broken camera then produces a photograph which will reveal the information as it is in that moment of time, such as a person in their current situation.
Produce live psychic videos on a television. It can either randomly display different TV channels to formulate a coherent sentence which will show a person's thought or film a place, even the most improbable ones such as the interior of Joseph's brain.
Analyze machinery to see if it has been tampered with in some way as he did with Telence T. D'Arby’s video game cartridges and console.
Conjure a map out of dust on an unpaved road, with precision on the location of a desired item.
Joseph has shown the ability to transmit the Ripple through Hermit Purple as an extension of his body. Joseph has used this property of Hermit Purple to attack DIO or prevent him from touching him by wrapping himself in it.
Hermit Purple's thorns can briefly be seen in the background of the second opening of the anime, "BLOODY STREAM". This acts as foreshadowing, as the opening is for Battle Tendency, and Joseph does not develop Hermit Purple until the following part, Stardust Crusaders.
However, when speaking about Joseph Joestar in JoJonium Vol. 9, Araki states "If Jotaro's group time-traveled to the world of Part 2, they would probably be able to see Hermit Purple wrapped around Joseph just like in Part 3."
Since Hermit Purple belonged to the previous part's protagonist, Araki decided not to make it combat-oriented or have a humanoid appearance so it wouldn't stand out too much.