First of all, this SCP 'occupies' dolls. At first, it starts with traditional bisque-ware porcelain dolls, then expands it's targets to synthetic dolls (like barbies and american girl dolls), then eventually to plushies and ragdolls (which it doesn't like because they can't really do much). Just to clarify, it only occupies one doll at a time. No residual affects on the doll after it leaves it (The Foundation may not know that though).
Of course, dolls are incredibly common in horror, and I know there are several on this site. So that already takes down it's quality. And "creepy dolls" are of course the final item on The Big List of Overdone SCP Cliches. But honestly, I can't think of any better vessels, and I personally wouldn't find it creepy.
The SCP would be an unknown entity that seemingly cannot be destroyed and cannot be contained. Which is cheesy. I feel like the inability to be contained has lost it's impact in the SCP world. But maybe I'm wrong. To clarify, the dolls can be destroyed, even while the entity occupies them. For all anyone knows, the entity CAN be destroyed, and just leaves the vessels before that can happen.
The idea with the SCP is that the entity controls dolls for usually extended periods of time. It typically chooses the doll right after its creation, and occupies it as long as it's deemed 'useful'. It won't reveal itself until it has an owner and has privacy with said owner. This owner could be any age, but the entity prefers children, and is more inclined to stay around children longer.
The entity's only goal is to help whoever it's owner happens to be. Typically, this is by talking with the owner about their problems, and offering gentle guidance with emotional problems. In more extreme cases, where the owner is in immediate danger, it may take action (within its abilities of course, more on that if there is any interest).
The entity doesn't understand the Foundation's motives and attempts to contain it. When the Foundation first finds it, it's like 'Ok new owners cool', stays around for a bit, then when it realizes it can't really help anyone, then moves on to the next vessel. When it keeps finding itself there again, it gets confused. As the Foundation continues recapturing it, the SCP goes through various stages of thought, like 'I'll tell you about myself, sure', 'I'm back here? Is there something you need?', 'Ok, seriously, this is getting annoying', and eventually 'LET ME GO, I HATE YOU'.
I think what originally lead to this idea was wanting to see how far the Foundation would go to try and contain something like this. It's like trying to catch smoke. Would they give up? Or would they take in any vessels they found, knowing they'd be abandoned once the entity knew what was happening? Would they hold on to the dolls, or discard them? I don't know.