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 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.
Vines(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.
Conjure a map out of dust on an unpaved road, with precision on the location of a desired item.
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.
Hermit Purple hijacks a plane's electronic system
Hermit Purple seems to be able to interface and tamper with electronic systems beyond its ability to divine information. Joseph uses this ability once to take control of a Cessna's electronic system and force it to stabilize. He also uses Hermit Purple to check Telence T. D'Arby's video game console to confirm whether it has been tampered with or not.
Joseph has shown the ability to transmit the Ripple through Hermit Purple as an extension of his body. Joseph utilizes this property of Hermit Purple to attack DIO or prevent him from touching him by wrapping himself in it. In the anime adaptation Joseph uses this ability to transmit the Ripple in the encounter with Steely Dan. When Lovers exits his brain, he uses his Ripple to kill all the growing flesh-buds.
Because Hermit Purple belonged to the previous part's protagonist, Hirohiko Araki decided not to make it combat-oriented nor have a humanoid appearance so it wouldn't stand out too much. In fact, he wanted to make Joseph's ability fitting for his role as a navigator. He was inspired by both telephone lines or network cables and the fact that a guide would use "lines" that connect them across the world to receive information; therefore, he made the Stand's vines represent those "lines". Hermit Purple's color was chosen because, according to Araki, purple is a classy color and easy to make work with older characters. He figured that purple would work better with an old Joseph rather than someone younger.
With Joseph returning in Part 3, Araki wondered about how to visualize the Ripple in terms of a Stand. The Ripple is life energy that spreads throughout the body, and Hermit Purple's vines are meant to be an explicit visual expression of such, by literally wrapping itself around Joseph's body.
Hermit Purple used for spirit listening through TV
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 he is much older in the following part, Stardust Crusaders.
However, when speaking about Joseph Joestar in JoJonium Vol. 9, Araki states "If Jotaro and the rest of the Stand-using crew went back in time fifty years prior, maybe you could have seen a younger Joseph using Hermit Purple."