Umbrellas are the ultimate breathable rain gear and great for those humid drizzly days where the choice is get wet from drizzle or get wet from sweat in your rain gear. They can be handy in camp while cooking, etc.
I don't like the idea of fastening it to my pack and just use one pole when in umbrella mode. Use any medium size umbrella to test, but it's pretty hard to beat the Golite for weight/cost/function. Birdiepal makes some good ones. Montbell makes one that looks like a big cigar when stowed---- really small. Shedrain makes the Shedrays silvered sun umbrella with wind vents in it (works for rain too). Stows to 12" but it weighs 13oz. You can find them at places like Target and Walmart.
I carry the Golite in a side pocket and under a compression strap like a tent pole. In fact, I just swap the umbrella with my spare pole.