Board Thread:Anime/Game Discussion/@comment-30778594-20180407013411/@comment-1739368-20180407023123

I imagine it's similar to ghosts in that Ripple is not a Stand, but it takes on a visually different form to Stand Users due to their higher "supernatural presence". When Joseph's Stand unlocked, this is the form it took.

That said, the concept of Stands was likely developed after part 2, or at least wasn't implemented during its publication. However, the canon states that Stands have existed from ancient times, but are usually treated as "physic powers" in most parts of the world. Retcon aside, if we had to make up a reason Joseph's "Hermit Purple" wasn't visible to us, it'd probably be because we weren't viewing the characters from the eyes of Stand Users. There's a certain scene at the start of P3, where it shows the cops being unable to see anything going on during Jotaro and Avdol's fight. This is likely how we "see" during the first two parts.