Key information we are given about Joel and Malthace in the opening chapter of this book concerns the fact that they are both twins. In addition, we know that they, unlike Daniel, come from a rather wealthy and well-to-do background, as their father is a rabbi in Galilee. Although Malthace is more cautious than her brother, it is clear that both are brave and valiant individuals to come to an area which is known to be populated by thieves and robbers (or freedom fighters as they style themselves).
One key difference is that Joel is extremely passionate about seeing the liberation of Judea from under Roman rule, whereas Malthace is naturally a lot more cautious about expressing her feelings about this. The common hatred of the Romans that Joel and Daniel share is of course something that initially helps them to form a friendship that will drive the rest of the plot of this excellent story.
You learn that Joel and Thacia are twins. You learn that they have a good relationship with each other and are really close. You learn that they are kind and thoughtful, because they offer food to Daniel. You learn that Joeal is training to become a rabbi. Joel and Daniel become friends.