User blog comment:27finnsprincess/The Talk About Marcy and Ice King.../@comment-75.143.157.172-20121017045408

I don't believe Marceline and Betty is the same person as you stated above, but I'm sure you know that by now. Marceline's growth rate is slower than humans, which is why she's 1000 years old like the IK now, but only 20 looking. He left her probably around the time she was still little, since in Memory of a Memory, she is seen as a preteen but her real dad is with her.