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Megalovania is not Chara's theme, of course.
But Megalo Strike Back is not Chara's theme, either.

Megalo Strike Back is unused but I think Toby Fox said it was SUPPOSED to be chara's theme

No, Megalo Strike Back was just intended to be a remix of Megalovania.

once upon a time is chara’s theme

No, Once Upon a Time is the theme of the Undertale, the story about the events of Undertale.
Just because the opening cutscene, in which Once Upon a Time plays, shows us Chara going up Mt. Ebbot and falling into the Underground, doesn't mean Once Upon a Time is Chara's theme.

no no no, Undertale (the song) the theme of Undertale, its literally in the name. in the song, you hear his theme, which is asriel’s theme, and once upon a time, which should be chara’s theme since the story of the game largely depended on those two

Undertale is Asriel's theme, since when Undertale plays the monsters tell you the story of Asriel.
Memory is both Asriel and Chara's theme.

HIS THEME IS ASRIEL'S THEME

Asriel has multiple themes.

No the MELODY of his theme is Asriel’s theme like in memory, and Undertale

I never said it wasn't.

LETS JUST SAY CHARA AND ASRIEL HAVE MANY THEMES AND THEN MOVE ON