Birch is a boy living Penn's nightmare. He didn't have a family that could protect him from the Warden's Commission, and he didn't have anywhere to run when his family was in trouble, so he's been raised in captivity because his sisters were all touched with special abilities, the same as Penn's. He doesn't even know his own birth name, as he was given the nickname "Birch bark" as a child and it stuck.
When they first meet, Penn writes him off as someone spoiled and weak, assuming that he's an officer's son because of his appearance, but she soon learns that none of her assumption bear out. Like many of the cast-offs taken in by The Show, Birch's early life was miserable. Seen as not-quite-human by an organization that's decided everyone born into a touched family has more in common with aliens than Earth, the fact that Birch is male makes him a particular fascination of those curious to see how he differs from sibling-sets that are solely female. And he's got the scars to prove it - both on his skin and in his memories.
Birch has also cultivated the same sort of public docile behavior many caged animals adopt out of self-preservation, which is something that irks Penn to no end. She wants to lash out, but he's been in the system long enough to know that direct attacks often end in more pain. It's not easy for the two to get along for long because their natures are so different, but eventually Penn comes to understand that these differences can be strengths. He becomes the temper to her steely nature, and tempered steel is much harder to break.