Alter

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late Middle English: from Old French alterer, from late Latin alterare, from Latin alter ‘other’.

wiktionary

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From Old French alterer (French altérer), from Medieval Latin alterare(“to make other”), from Latin alter(“the other”), from al- (seen in alius(“other”), alienus(“of another”), etc.; see alias, alien, etc.) + compar. suffix -ter.

Probably from alter ego.

alter (plural alters)

etymonline

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alter (v.)

late 14c., "to change (something), make different in some way," from Old French alterer "to change, alter," from Medieval Latin alterare "to change," from Latin alter "the other (of the two)," from PIE root *al- (1) "beyond" + comparative suffix -ter (as in other). Intransitive sense "to become otherwise" first recorded 1580s. Related: Altered; altering.