Alanna's twin brother Thom looks just like her, with red hair and purple eyes. Plus they both have some magical ability. But that's about it for similarities. As Maude observes, "The only person in the world Thom loved, beside himself, was Alanna" (1.24). In contrast, Alanna … pretty much loves everyone except herself, at least until the end of the book.
In a lot of ways, Thom doesn't seem cut out to be a nobleman. Take manly tasks: while Alanna loves to hunt and ride, Thom has to be ordered to get his bottom up on a horse. Coram even worries about how this will reflect on him: "People would see that Thom was no warrior, and they would blame Coram—the man who was to have taught him the basics of the warrior's craft" (1.72). Coram may care, but, from what we know of Thom, though, he probably doesn't care what others think of him so long as he gets his way.
Of course, we don't really know too much about him. We only see him at the very beginning of the book; otherwise, it's a just a long-distance relationship, with him writing letters in to Alanna occasionally. We learn from one of those letters that he plays stupid in his sorcery training, so as to avoid drawing attention to himself. What's funny is that, while it seems at first that not wanting to draw attention to himself is just one more trait he shares with Alanna, it actually shows us that Thom is, well, deceitful and conniving—two things that Alanna would never be.
Thom also has a high opinion of his abilities, writing, "I know a lot more than they think I know" (6.250) and "Give me a few more years, and we'll give your smiling friend a run for his money" (6.252). So, yeah. Basically, we want to like Thom because Alanna does, but we're not sure if he's got a good heart or not. We'll have to wait and see how he turns out in the next few books.