Julia Bertram is a bit of a mystery. For much of the novel she functions as Maria's shadow. She's always second-best: she loses Henry to Maria; she loses a part in the play to Maria and then to Mary. And she even loses the chance to return home once news of Maria's scandal breaks. In a panic, Julia ends up eloping with the man who paid her attention when Henry did not: Mr. Yates.
The problem with Julia is that we never seem to get enough information about her. We hear that she's traveling with Maria and is having a great time in London, but we never know what exactly she's up to. Julia also rarely speaks. When she does, she is intriguing, though.
Julia, who had scarcely opened her lips before, observed, in a sarcastic manner, and with a glance first at Maria, and then at Edmund, that "the Mansfield theatricals would enliven the whole neighbourhood exceedingly." (15.61).
We do get a few narrator descriptions of Julia that give us a little insight into her personality, at least: that she has a quick temper, that Henry broke her heart, that she recovered from Henry better than Maria, that she hates being at Mansfield Park as much as Maria does, and that she can be rather selfish. In the end, though, Julia is still a bit of a mystery. This may be because she lacks a close relationship with our two main characters, Fanny and Edmund, but Julia in the end still seems like she has some untapped potential.