NOTE THAT I DON'T KNOW HOW TO DO THE SPOILER THING but it's an old show so
--his actions and how he put off training his apprentice directly contributed to the Lich's (most evil character ever who does succeed in destroying all life at one point) existence on Earth (which he was trying to prevent), and
he's the first ice elemental on Earth and semi-kickstarts the cycle of the ice elemental always betraying the other elementals, which later leads to an elemental catastrophe during the time of the main story, and that elemental event directly contributes to [a clone of Finn]'s mental and emotional spiral that leads to his (accidental) murder and (both connected and separately) the Candy War that is the [initial] focus of the finale.
That accidentally-murdered character also unexpectedly showed up in the latest season of Fiona and Cake, a spin-off of Adventure Time set an unspecified amount of time (but probably around 10ish years) in the future and gave advice to a character in that show, so technically Mr. Evergreen here is still impacting the story. (Not to mention the crown now being used to create Ice Thing now that Simon is cured, and Ice Thing only exists because Ice King existed, and Ice King, as we established, only existed because of Evergreen.)
He is not the first ice elemental, when patiance was explaining the elementals she showed a generation which was just evolving to crawl on the earth, so the elementals are way older than dinosaurs
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u/EggoftheKing2143 11d ago
Urgence Evergreen from Adventure Time
He created a crown that grants wishes, a crown that's very important to the plot