Ridley's stand would be fairly simple. A creature ranging from half a meters tall to several, with a ridiculous stand density causing them to be constantly visible due to his amount of vital energy. Compared to most stands with one form, Ridley's has three Steps: A small, humanoid-ish child that looks very human save for lacking a nose and having flat areas where ears should be - a small blonde child, specifically - A bio-mechanical brain covered in spines, drool and eyes, and a Proud, brilliantly plumed bird that is reminiscent of an Archaeopteryx.
These three switch partially related to Ridley's mood, with the small child being present when Ridley is at his most cunning and calm, the brain taking form when Ridley has a mind for science and is enthusiastic regarding a project he's been working on, and The brilliantly plumed bird taking shape when Ridley is at his most violent, destructive, and wrathful. Ridley cannot choose which stand he has out consciously, so to keep a desired form out he must maintain a certain mood.
the small child form is capable of acting very sweet and innocent, and using this to lure people into a sense of false security. though it's power does not help them in this attempt, it's a born actor and is a good actor.
it's actual ability, however, is to drain the energy from those it touches, slowly but surely. It requires skin contact to do this, but drains slowly enough as to be subtle, making no show of the fact it's draining this energy. this energy is given to Ridley, potentially allowing him to heal his wounds and eventually can kill someone as they lose the basic energy to maintain vital functions. The stand has a surprising amount of strength for a kid but could be overpowered by a very physically fit adult with some effort.
It is 'Fuel'.
The brain is capable of acting on it's own but is more directly controlled by Ridley. it does, however, have a high effective range of up to 200 meters. it's only offensive comes from the fact that it can float and rocket into things, smacking them against it's spines, but it is not a match for any true combat stand. what it can do is figure out the mechanics of devices it spends even a second or two observing, allowing him to understand the mechanical function and use of what it gazes on. Combined with several tentacles the Brain can use to do precision work with high speeds, it can quickly turn any robotic or mechanical device to it's own ends and turn a mechanically advanced city into a nightmare.
it is 'that which I desire'
The last stand is quite simply a bomb. restricted to a range of ten meters, this bird's power is to explode on contact with opponents, leaving Ridley and itself unscathed but creating explosive power that it can repeatedly dish out. This power can also be operated on contact with the ground, and it's speed and durability are surprisingly high
unsurprisingly, it's name is "Fire".
Ridley's stand's general name is, of course, "Fuel", based on the metallica song.