She is in these episodes:
~Evicted!
~Henchman
~It came from the Nightosphere
~Memory of a memory
~What was Missing
~Marceline's closet
~Daddy's little monster
~I remember you
~Five more short graybles
Chat with our AI personalities
There are a few websites out there where you can view Adventure Time episodes, including Youtube. But if you want all episodes for free, try watchcartoononline.com or Mastertoons.com (try to use watchcartoononline.com if you want to avoid endless ads)
That would be Olivia Olson. She also voices Doofinsmert's daughter, Vanessa on Phineas And Ferb, which is a coincidence because Marceline's dad is evil to. Olivia always has to play that kind of role,it would seem...
Im not sure but I calculated views of videos ( not all) and got over 24 million
See the related link below for a full list of the episodes.
BMO is genderless,I know you might say "but in conqeust of cuetness he says"m-lady" or in "what was missing" marceline says "come here boy" but BMO is confirmed genderless by the makers(note in the fiona and cake episode he/she does not change at all)