Hannah Levine is the niece of Velcro Horns trombonist Dan Levine. At the age of 11, Hannah sang the lead vocal in the song "One Dozen Monkeys" on Here Come The 123s, and provided vocals for "I Can Add," "Ooh La! Ooh La!," "High Five!," and "Zeroes." Later, she also contributed vocals to "Computer Assisted Design" on Here Comes Science. In 2008 and 2009, Hannah occasionally joined the band live to provide vocals for "One Dozen Monkeys".
In an interview with Zooglobble, John Flansburgh had this to say about Hannah:
She is, and I mean this in the most positive way, a professional kid actor and singer. She's got an agent, and she seems to work pretty regularly for a kid in school. She can do line readings any way you ask, and takes direction with an adult's level of comprehension. It was really amazing. I also believe she is lucky because both her parents are very hard-working performers based in New York—not LA—and I don't think she is being raised with any big fantasies about what "it" is that she is doing. I get the feeling that performing to her is just an interesting part of her life, with school and everything else. It's fun. I've seen my share of stage kids, and honestly I'm not a big fan of that scene. I am happy to report she has got a seriously good handle on it.