Bird is Alma's younger brother. He gets the name "Bird" by jumping off a building, in an attempt to fly. Later, he falls off another roof and sprains his wrist. ("A" for effort there, Bird.)
At nine years old, Alma tells us, Bird became a devout Jew. He claims to be a lamed vovnik—in Jewish scripture, one of the thirty-six people in the world who may become the Messiah (although sometimes Bird thinks he might already be the Messiah).
Bird provides a sharp contrast to both Alma's search for identity and her quiet grief. He represents the impulse among young Americans within the Jewish Diaspora to connect to their heritage through religion.
Alma, meanwhile, is horrified by her brother's eccentricities. She worries about him being bullied and not having any friends. She also frets that the exaggerated stories she's told Bird about their father have caused his delusions of grandeur.
Finally confronting him, she demands that he "stop talking about God" and "stop making weird noises, and jumping off things [...] and stop wetting your bed" (13.22). Her advice is to "push your feelings down and try to be normal" (13.22). This has apparently been her own strategy, as we notice throughout the book. But instead of really following Alma's advice, Bird ends up opening his sister's eyes to her own isolation.
Bird's religious faith stems directly from a desire to connect with his deceased father. It's finding his father's copy of The Book of Jewish Thoughts that begins his conversion, after which he befriends Mr. Goldstein, the janitor at his Hebrew school (which seems like a thinly veiled attempt to find a father figure). Bird only becomes more isolated as he starts to spend much of his time speaking to God, convinced that He can hear him.
Bird spends most of the book off-stage. His main activity, as far as we know, is selling lemonade at a lemonade stand—only later do we discover he's saving up that money to fly to Israel. But he's also building an ark out of garbage and scrap cardboard because he believes a Biblical flood is on the way and he wants to make sure his mother and sister are OK.
When things start going bad for Bird (it stops raining, the ticket to Israel costs more than he expected, Mr. Goldstein gets sick), he begins to doubt his status as a lamed vovnik. In addition, he finally seems to recognize the questionable ethics of some of his actions—like, say, stealing from his mother's wallet—that before he had justified as being part of God's plan (which is an interesting statement by Krauss about religious zealotry).
Now finding both faith and doubt within himself, he decides to test his status as a lamed vovnik by actually trying to help people. He discovers that Alma is searching for someone, and even though he misunderstands whom exactly she's looking for, he provides the connection that helps her find that person by the end. Long story short: it's through Bird that Leo and Alma finally find each other. That's some good work, lamed vovnik or not.