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THE SIMILARITIES:
They both disguised themselves up until the climax of their respective sources (For Sarv it was a nun and for Melvinborg it was the school superintendent)
They're both simped for by their respective fandoms
They're both easily recognizable
They both are associated with a food item (Sarvente's associated with Neapolitan Ice Cream and Melvinborg is associated with blueberry muffins)
Tying to the second point, their creators know that people simp for them (For Sarv, I know since Dokki.doodlez, Sarv's creator, once made a bunch of doodles of Sarv's demon form since people simp for Sarv and as for Melvinborg, I assume because he was shown to be shirtless in one episode)
They both have younger selves that have canon designs (Melvin and Young Sarv being said younger selves)
They both die in some way (For Melvinborg it was the altered future and for Sarv it was the broken vow)
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Knowing all of this as both a MFM and TETOCU fan, I CANNOT be the only one who sees these similarities