Buddy describes his friend in somewhat minimal terms, but his descriptions are all the more poignant because they are spaced out throughout the story. To introduce her, the narrator (Buddy, who Truman Capote based on his childhood self) says she has "shorn white hair" and is "sixty-something." He also describes a long childhood illness that left her shoulders hunched, despite being "small and sprightly, like a Bantam hen," and we know she is energetic because she is able to get Buddy to do things even when he is tired or disinterested. Her face is "remarkable" and "craggy" like Lincoln's, but also "delicate" and "finely-boned," with eyes that are "sherry-colored and timid." By using these very specific and unusual terms to describe his cousin, with words that are visual and descriptive, as opposed to vague and trite, we get a very solid picture of Buddy's cousin.
Buddy also describes his cousin's personality throughout the story, but rather than stating this in general ways, he lets her behavior speak for itself; this is the mark of a good literary story, one that shows rather than tells. She gets him motivated to help her start make fruitcakes (their yearly ritual) by exclaiming several times, "it's fruitcake weather," and he sees a "purposeful excitement" in her eyes. We know she is emotionally sensitive, and see this in her response to being chastised by the other adults in the household when she gives Buddy some whiskey to drink: "My friend gazes at her shoes, her chin quivers, she lifts her skirt and blows her nose and runs to her room." Note that we detect her emotional state merely through physical actions, not by being told she was "sad" or "upset."
Perhaps the most telling description of Buddy's best friend is one of the story's most unembellished sentences, in which Buddy describes her as "still a child." With this sentence, we understand why the other family members treat her poorly, and why she and Buddy have such a close bond.