Frederick isn't in the book for very long, since he only shows up at his mother's death. But he still plays a big part in the plot of this novel. For starters, he is constantly on the minds of his family members, and for this reason, he has a huge influence on their personalities. His mommy thinks of him as an angel. But when we finally get a chance to meet Frederick, it's clear that he's just a normal guy.
He's definitely not the angel everyone makes him out to be. He greets his younger sister Margaret (after not seeing her for years) by saying, "But, Margaret, what a bungler you are! I never saw such a little awkward, good-for-nothing pair of hands. Run away, and wash them" (2.5.35). There is absolutely no one else in this book who Margaret would tolerate this kind of tone from. But he's a big brother and gets to kid around. Also, Frederick won't be around for long.
As the narrator tells us, Frederick has "delicate features, redeemed from effeminacy by the swarthiness of his complexion, and his quick intensity of expression" (2.5.39). In other words, there's a delicacy about him, but also strength. The guy is definitely a doer instead of a thinker, as he says himself: "Thinking has, many a time, made me sad, darling; but doing never did in all my life" (2.5.52).
This philosophy may or may not play out in Frederick's favor, since it is this kind of thinking that made him lead a mutiny and get himself chased out of England. Maybe if the guy thought a little more before acting, he wouldn't be in the mess he is. But at the end of the book Frederick is getting married to an awesome Spanish woman, living in Spain and (we imagine) chilling on a beach and eating paella every day. That sounds pretty good.