Walking With Misty?
If this is the case, it means the one most likely to be convinced to do what you want, without any real insinuation of stupidity. Here is means because she was kind-hearted, rather than foolish or gullible. (Perhaps with a hint of needing something to do; a distraction.)
Generally a sucker is someone who can be convinced to do somethig that will give someone else an advantage. Modern scams could be described as a scammer and a sucker.
As Olivia says, "in the park" refers to an arbitrary group. (figurative)