Oh nice I really like them! Revolution is one of my favorite songs by them cx 
There was another song I found too but now I can't remember what it was XD 

Edit : I found it! And a couple others xD

[i]Leave the City[/i] by twenty one pilots 
Ok so mini spoiler for Penelope too but she doesn't just give up right after the mission fails. A month or so after she actually tries to track down the staff with William but it obviously doesn't pan out. I imagine this song sorta fits the mood when she is really coming to terms with having to let Crow go.
[youtube]https://www.youtube.com/watch?v=kf6o4D5b6HY[/youtube]

[i]Morph[/i] by twenty one pilots
This reminds of Penelope after the timeskip and just adjusting back to her old life. 
[youtube]https://www.youtube.com/watch?v=JcycFPx_X14[/youtube]