The ending of Luna is pretty dramatic. After a big blowout with their dad, Luna decides to go to Seattle in order to start her life and transition. As Regan watches her board the train, she says a final goodbye to the brother she once had:
"Good-bye, Liam." I spoke the words aloud so their music filled my head: "Hello, Regan." (26.168)
This is a particularly poignant moment because Regan isn't just saying goodbye to Liam for the time being—if all goes according to plan, she'll actually never see Liam again. Her brother will have disappeared forever and in his place, she'll gain Luna, her new sister.
Regan has been so bogged down by Luna's problems that she's been unable to live her own life, and so in letting Liam go, she's also able to say hello to herself and start working on her own individuality. Now that Liam is leaving, Regan will have to live out her own life and figure out what she wants to do and who she wants to become. By leaving, Liam is allowing Regan to actually grow as a person—which is a pretty incredible gift.