Cardenio is your typical hopeless romantic, maybe even more so than the big Don Q.—and that's saying something.
It's actually pretty funny to see this guy living in the wilderness and surviving on twigs and berries just because he lost his beloved Lucinda. As he tells his listeners, "I will, and can have no health without Lucinda; since she forsake me, I must die" (22.214.171.124). The lengths he'll go to for love make some of Don Quixote's strange adventures seem a little less far-fetched, and it's almost as if Cervantes is using Cardenio to say to us, "Hey, people do crazy things for love; so is Don Quixote all that unbelievable?"
That whole living-on-twigs-and-berries-in-the-wilderness-because-of-unrequited-love thing is standard fare in medieval romance, by the way. Love was serious business back then: it could make you crazy, and one way to express it was to wander to around in the wilderness to suffer. Like Don Quixote, Cardenio is probably acting out stuff he has read in books.
When we first meet Cardenio, the first thing he says is: "the place of my birth one of the best cities in Andalusia; my descent noble, my parents wealthy; but my misfortunes are so great, that they have doubtless filled my relations with the deepest of sorrows" (126.96.36.199). Like most characters in this book, Cardenio introduces himself by mentioning the most important piece of information about him, his social class. He comes from a wealthy background, which automatically makes everyone treat him more kindly. It's a shame, but that kind of favoring of the rich would have been super entrenched in 17th-century Spain.
But for all of his good breeding, Cardenio is a violent dude when he doesn't get what he wants—which would be Lucinda. He is known to attack people without cause (hey, sort of like Don Quixote), and when he's chatting with Don Quixote at one point, the narrator tells us, "being provoked by these abuses into one of his mad fits, he took up a great stone that lay by him, and hit Don Quixote such a blow on his breast with it, that it beat him down backwards" (188.8.131.52).
Cardenio almost bashes the Don's brains out before he finally takes off back into the wilderness. Once he's reunited with his beloved Lucinda, of course, everything seems to be fine. You have to wonder, though: what's up with all these dudes beating each other up over half-imaginary ladies?