It is generally understood that there are two separate "childhood friend" characters; the "girl next door", who has known the main character since childhood, and the "mysterious newcomer" who, usually by transferring into the protagonist's class, disrupts his life with claims of love and/or marriage promises. It is the first type that I am setting out to examine.
Within this first archetype, too, there are divisions; there are the obvious categories of the "tsun-tsun" character and the "dere-dere" character, which provide us with a reasonable starting point. For example, Sohara Mitsuki of "Sora no Otoshimono" is clearly on the "tsun" end of the scale, with Nanami of "Lamune" providing reference on the other end. However, I belive that the archetype can be divided on another axis: that of "caring vs. cared for".
Consider, for a moment, the other way these two characters contrast. Sohara looks after the protagonist, cooking for him, helping with daily chores, etc. By contrast, in "Lamune", Nanami is cared by the protagonist who looks after her as though she was his little sister, adopting the caretaker/protector role. There is a clearly different relationship between the main character and the osananajimi in these cases--but in other ways they are similar. The same relationship, the platonic closeness that borders on love, is present in both. It is this relationship that defines the osananajimi: the "almost love", and the fear of losing the certainty of their platonic relationship.
This, then, is the basis of my distinction: the "tsun->dere" and "caring->cared for" axes form a cartesian coordinate system, and by plotting where each childhood friend character falls within it, their characteristics, and whether or not they end up in a romantic relationship with the main character, I hope to gain greater insight into the childhood friend as an archetype, and said archetype's purpose and meaning.
-->Numbers and charts here<--
-->A more detailed musing on the role and purpose of the osananajimi<--
-->Click here for Dragoonkain's excellent rundown on basic concepts and his list of relevant series<--