'appearer' is formed by adding the agentive suffix '-er' to the verb 'appear', which comes from Latin 'apparēre' via Old French and Middle English. 'apparēre' meant 'to come into view'.

'apparēre' (Latin) → Old French aparoir/apparoir → Middle English appear(e) → modern English 'appear'; the agentive noun 'appearer' developed by adding '-er' to the verb in English formation patterns.

Initially the root meant 'to come into view' in Latin; over time it broadened in English to include 'to become visible' or 'to seem', and the derived noun now means 'one who appears' or 'one who makes an appearance'.