A guerrilla was first a war, not a soldier. The Spanish word means "little war," and for a while English purists tried to keep it that way.
Spanish guerra means "war," and the ending -illa makes things small. Etymonline defines the Spanish guerrilla as a body of skirmishers or skirmishing warfare. The fighter had his own word, guerrillero.
The word reached English during the Peninsular War of 1808 to 1814. Napoleon's armies had occupied Spain. After the regular Spanish armies were beaten, bands of civilians kept fighting in small, irregular groups. Etymonline describes them as peasants and shepherds who harassed the occupying French.
Merriam-Webster dates the English noun to 1809 and the adjective to 1811. Wikipedia credits Arthur Wellesley, later the Duke of Wellington, with adopting the term in 1809, using it for the individual fighters. That usage is exactly what the purists resisted. Etymonline notes that they tried to keep guerrilla for the style of warfare, leaving the fighter to guerrillero. They lost. Today a guerrilla is the person, and "guerrilla warfare" names the method.
The deeper root holds a surprise. Guerra is not Latin. Latin had bellum for war, but Spanish, Portuguese, Italian and French all borrowed a Germanic word instead. Etymonline says one reason may be that bellum was too easily confused with bello-, "beautiful."
That Germanic word, related to Old High German werra, "strife," also reached English. It arrived through Old North French werre and became war. So guerrilla and war are cousins: one Germanic word, reaching English by two different roads.
The spelling still trips people up. Etymonline calls "guerilla," with one r, a common misspelling that may now count as a variant, as in French guérilla. The word has nothing to do with the gorilla, which comes from a Greek account of a voyage along the African coast.