In my opinion, these two are definitely Finn and Jake's past lives.
And it's really sweet if you think about it, both Marcy and PB have been friends with Finn for a really long time.
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In my opinion, these two are definitely Finn and Jake's past lives.
And it's really sweet if you think about it, both Marcy and PB have been friends with Finn for a really long time.

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Been wondering if Marceline's acoustic bass was a gift from Two Bread Tom
Especially considering her Ax Bass was broken during her fight with the Vampire King and Tom's tribe preferred Marceline's playing to his. Maybe it was token of appreciation for protecting them or given as something to remember them by for when they sailed away.
Adventure Time: Jake the Brick, Everything Stays, Astral Plane & Vamps About
Old man yaoi u say?
Oh lawd he survivin’ 😔
For context Finn is 12 here and is carry the stake for defense and isn’t actively hunting any vamps yet :)
He starts to after meeting Two-Bread Tom tho, this is also before the whole “half-demon fiasco” (see timeline 😉)
the human survivors and the illusion of safety!!!🤷♀️
Cause I read islands recently and Jo and two bread tom are underrated imo

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Marceline and the Founders
Quick thought I've been thinking of recently (since I've been getting back into Adventure Time as of late due to Fionna & Cake) but I wonder if the humans of the Island ever discovered Marceline's connection to the Founders and what the consequences of that might be.
Marceline knew the founders as actual people rather than the deity-like figures which Island propaganda appeared to portray them as. So her insight could very much go against some very fundamental tenants of official Island history.
Of course, the very fact that the humans of the Islands have returned to Ooo could indicate they no longer take "Founder's Truth" to be the gospel in regards to running their society. Or perhaps they tried to rationalise it.
I have seen some discussion on reddit before regarding how aware the Islands are of Marceline's existence. Personally I feel she likely has a limited by important place the "mythos' of the Founders. The fact she was turned into a vampire and so parted ways with the Founders before they actually founded Founder's Island probably meant she's not as prominent as other Founders like Two Bread Tom who would've taken a much larger leadership roll in early settlement of the Island.
However, while her time with the Founders may have been relatively short (Jo, aka the girl with the bunny hat, didn't physically grow up much between the time Marceline first met her and when she fought the Vampire King), she did still appear to have grown quite close to them. And I do feel they likely would have honoured her sacrifice in fighting the Vampire King so they could escape the mainland.
I do think that others are correct though that her half-demon nature is likely unknown by modern humans, both due to details being lost in time and also because a friendly non-human (well non-complete-human) would go against the propaganda of "everything outside the Islands is out to get Humanity so we can't leave" stuff. Personally I have also had the thought she could be considered the predecessor to the Guardian, since that was basically her role in their group. Although for the purpose of propaganda, her effectiveness as a guardian may also have been played done and/or her sacrifice got the most focus.
So perhaps the humans of the islands just recall "Founder Marceline" as someone who sacrificed her life for the other Founders, a haunting example of why humanity must never leave the safety of the islands or stray from the Founder's Truth...
Of course, given the aforementioned deity-like status of the Founders, that does make me kinda curious of how the Islanders would react to learning that one of the Founders is still around. Certainly wouldn't be the only time Marceline's gets canonised by a saint by folks she helped saved....
Could also definitely be interesting to see what Marceline's on reaction might be to learning of the ultimate fate of that band of survivors she once was a part of, and also how she went down in history.
Perhaps the humans of the Islands might try seeking our Marceline for wisdom and leadership if they knew, given the importance of the "Founder's Truth" as being treated as infallible for so long, old habits die hard after all. Maybe might lead to a dynamic akin to this...
In all seriousness though; someone like Marceline, who they can trust more or less, probably would be beneficial in helping humans re-establish themselves in Ooo after being absent for so long.
islands (2016).