Kidnapped at birth and raised by a man claiming to be their father, Arviragus and Guiderius (a.k.a. Cadwal and Polydore) sound like a book deal waiting to happen. With that history, they could be all confused and morally corrupt, but instead, they're kind, patriotic, and brave.
We don't meet these brothers until halfway through the play, but that doesn't stop them from making a big impact. In fact, with the help of their wannabe dad, it's Arviragus and Guiderius who save their real dad, Cymbeline, and his kingdom.
We didn't say it wasn't confusing.
These boys yearn for adventure. We see them begging Belarius for it early on, so it's no surprise that they step up to the plate when their country needs it most. By fighting for Britain and their real dad when they could have just stayed out of the whole thing, Arviragus and Guiderius show that they are real princes. They know when they are needed, and they don't back down from a fight: it's really just the two brothers, Belarius, and Posthumus who take down the entire Roman army when it looks like all is lost.
There's no doubt in our minds that the brothers are strong, valiant, and skilled fighters. But they're more than just foo-fighters: the boys also have a softer side.
When Imogen (disguised as Fidele) shows up in their cave, Arviragus and Guiderius welcome her in with open arms. They even call her "brother" before they know that they're all actually related (4.2.86)—it's clear that they care a lot about family, whether or not they know it.
Their relationship with Imogen shows us that blood is thicker than water—the water in this case being Belarius, who kidnapped them and raised them. It's not entirely clear what Shakespeare is up to here. Is he suggesting that Imogen and her brothers have some kind of innate family feeling just because they're related by blood? That's possible, given that Shakespeare seems to place an extremely high value on family relationships and family bonds, particularly in these late plays.
Or maybe we're supposed to understand that these three feel close to each other because they're all good, noble, and heroic people? Shakespeare isn't going to give us an answer, but it's worth thinking about as we read through this play.