PB can speak German, which she has 3 times in the series. She said "Ich bein heir" in "What Have You Done," "Auf Weidersein" to Finn over the phone in "Go With Me," She also something too quick to understand in German at the end of "What Have You Done?"
PB also have some sort of past with Marceline since she knows Marceline in "Go With Me" and Marceline even gave her a shirt that the Door Lord stole which means it is very valuable for her in "What Was Missing."
I think that PB must have some German decent because Earl of Lemongrab has a somewhat European accent. Plus, PB also speaks it. So she could have "immigrated" from Germany to Ooo.
Her reign in the kingdom must be short because she is 18 and as the Earl of Lemongrab stated, you must be 18 to be princess. So she could have came over shortly before her 18th birthday.
She probably made Earl because she was lonely in the castle but ended up making a mistake, she is apparently too busy in her expiriments to realize that she is lonely.
She might be Betty,Simon's girlfriend, she has a mix of Human DNA and Bubblegum and i have evidence: