"Influence" originates from the Latin influere, meaning "to flow into", which evolved from in- ("into") and fluere ("to flow").  It was also influenced by the Medieval Latin word influentia ("a flowing in") and the Old French influence (13th century) "emanation from the stars".  Dating to the late 14th century, the word originally referred to an "ethereal fluid" from the stars that flowed down to affect human destiny and character.  In the mid-1600s the meaning shifted to the power an individual or group has over another (social/political).  

According to the Oxford English Dictionary (OED), the noun influence was first used in English during the Middle English period (1150-1500), with the earliest evidence appearing around 1374 in the writings of Geoffrey Chaucer.  The verb influence appeared much later, with the earliest evidence in 1658, in the writings of Oliver Cromwell.  The earliest known use of the adjective influential is in 1570, appearing in a translation by John Dee, the noted mathematician, astrologer, and antiquary.  The earliest know use of the noun influencer is from 1664, appearing in the writing of English philosopher and theologian Henry More in A Modest Enquiry into the Mystery of Iniquity, referring to a person who exerts influence.  

A character using influence—whether to persuade, change, or challenge others—is crucial for driving the plot, exploring themes, and fostering character development.  Known as an "Influence Character", they often force the protagonist to confront flaws, alter their perspective, and grow, adding necessary dramatic tension and depth to the narrative.  

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INFLUENCE Definition & Meaning - Merriam-Webster

INFLUENCE Synonyms: 100 Similar and Opposite Words | Merriam-Webster Thesaurus

Of Influence | Definition & Meaning - Merriam-Webster

The Influence | Definition & Meaning - Merriam-Webster

UNDER THE INFLUENCE OF Definition & Meaning - Merriam-Webster