Karl Marx's lifelong collaborator, patron, editor, and — after Marx's death in 1883 — the chief organizer and interpreter of the tradition that bore Marx's name. Engels was the son of a Rhineland textile magnate and ran a Manchester mill for decades, using the proceeds to keep the perennially broke Marx family afloat. He also used his vantage point inside industrial capitalism to write The Condition of the Working Class in England (1845), a pioneering empirical study of urban poverty that foreshadowed both Marxist sociology and later social reform writing.
With Marx he co-wrote The Communist Manifesto (1848) and The German Ideology (written 1846, published 1932). After Marx's death he edited volumes 2 and 3 of Capital from mountains of difficult manuscripts — an act of scholarly devotion that also put him in a position to shape how orthodoxy read Marx. Engels's own later works, especially Anti-Dühring (1878) and Socialism: Utopian and Scientific (1880), systematized Marxist thought into something closer to a worldview, including the "dialectics of nature" — an extension of Hegelian dialectic to the physical world that later Marxists (Western Marxism especially) often regarded as a mistake.
Historians now argue endlessly about how much of canonical "Marxism" is really Engels's systematization rather than Marx's thought. Either way, without Engels, Marx's unfinished mountain of manuscripts would likely have sunk without trace.