How to Say "defector" in Spanish
The Spanish word for “defector” is “desertor” — B2 level.

Examples
El desertor dejó su uniforme cerca de la frontera.
The deserter left his uniform near the border.
Se convirtió en un desertor de su propio partido político.
He became a defector from his own political party.
Muchos ejércitos castigaban severamente a cualquier desertor.
Many armies used to severely punish any deserter.
The '-or' ending
Just like in English ('actor', 'director'), words ending in '-or' in Spanish usually describe a person who performs a specific action.
Using 'ser' with desertor
Since being a 'desertor' is considered a label or a status, you should use the verb 'ser' (to be) rather than 'estar'.
Don't confuse with 'postre'
Mistake: “Using 'desertor' to mean someone who likes dessert.”
Correction: In English, 'desert' (sand) and 'dessert' (sweet) are similar. In Spanish, 'desertor' only relates to 'desertar' (leaving/abandoning).
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