Mr. Birkway is Sal's crazy awesome English teacher. He loves everything about teaching, and he loves everything about his students. He's one of those great teachers who is enthusiastic about everything and thinks all his students are geniuses.
However, he causes some trouble when he decides to read everyone's journal assignments out loud to the class. Chaos ensues as the students figure out who wrote what about whom. It's clear he regrets being so revealing.
Over time, we learn that Mr. Birkway is Mrs. Cadaver's twin brother and Mrs. Partridge's son, which brings him even closer to Sal.
We suspect that Sal really, really likes Mr. Birkway, because he is the only teacher she bothers to tell us about. Plus, we learn, he reminds her of her mom: "There was a liveliness to both Mr. Birkway and my mother, and an excitement—a passion—for words and for stories" (19.21). It's no wonder, then, that Sal spends so much time telling us about Mr. Birkway.
Oh! And Shmoop just has to mention Mr. Birkway's totally awesome lesson on symbols. We think it just might come in handy for you. He shows his students a picture of a vase. Or is it two faces?
Then Mr. Birkway pointed out how you could see both. If you looked only at the white part in the center, you could clearly see the vase. If you looked only at the dark parts on the side, you could see two profiles. The curvy sides of the vase became the outline of the two heads facing each other. (32.60)
See, Mr. Birkway knows an awesome secret. Two people can look at the same exact thing and have wildly different interpretations of it. And the same goes for symbols. Ah, the ambiguity!