Forms represent the common part of many varied things. In Socrates' speech in the Symposium, he explains how to discover the form of beauty: looking at one beautiful thing, then another, at last you should be able to see what is common to them all, that is, beauty itself, not a particular beautiful thing.
Thus, there is no form of any particular beautiful person. There is a form of beauty that beautiful people draw on. Nor could there be an infinite number of forms for an infinite number of conceivable people. As demonstrated in Parmenides (in a different context), the existence of an infinite number of forms is fatal to the theory. There is no reason to believe that an infinite number of forms is the source of an infinite number of sensible things; everything that exists has to be attributable to a finite number of forms for the theory to have any explanatory power. There can't be a form for everything. We see this again at the very end of Timaeus, where the different types of animals are derived from only four different forms.
Nor could there be a form of an imperfectly-drawn square. The forms are perfect. When a person draws a square imperfectly, he is thinking about the perfect square but failing to make an exact imitation of it. The forms of the shapes are described in Timaeus. Triangles are said to be the source of solids and even the elements. These are not imperfectly-drawn triangles, but perfect triangles. So the form of an imperfectly-drawn square doesn't make sense. An imperfectly-drawn square is just that, a shape that is not drawn accurately according to the standard of the perfect square. Its existence is only in the sensible world, as an imitation of the perfect square that can be perceived in the mind. We can see the form of the square by looking at the "squareness" of things that resemble this perfect square, which reminds us of the square that is already known to the soul, according to Plato's doctrine that all knowledge is recollection (in Phaedo and elsewhere).