Identify the part of speech the underlined word belongs to.
He knows mebetterthan you.

Some words are used sometimes as adjectives, sometimes as adverbs.
Adjective: He is no better than a fool.
Adverb: He Knows me better than you.
Adjective: There is much truth in what he says.
Adverb: The patient is much better.
Adjective: This is a hard sum.
Adverb: He works hard all day.

Source: High School English Grammar & Composition by Wren & Martin.