Julia is the only stable part of Jason's home life throughout Black Swan Green, and even though Jason dislikes her at the beginning of the book, he comes to realize that they are really allies. In Relatives, Jason is being embarrassed at the dinner table when Helena brings up the poetry award he won. Their cousin Alex starts teasing Jason about it, so Julia stands up for Jason and insults Alex back. Jason is startled by this help from Julia, and thinks, "I could've kissed Julia out of sheer gratitude. Almost."
In Rocks, Julia and Jason seem to ally with each other during their parents' fighting. After one of their parents' big fights, it is revealed to us that Jason and Julia have their own system for ranking fights and they have bonded, even as their parents' relationship is starting to crumble. Jason also looks up to Julia as a voice of reason and power in Rocks. For example, in one section, Julia asks their dad what a Pyrrhic victory is even though she knows the answer because she is trying to make a point to her dad about the fights he is having with their mom. Even though Julia's position as a voice of reason is very prominent is Rocks, Jason looks up to Julia for information and as a role model throughout the novel. Jason often recounts things she has said as truth and he often is impressed with Julia's ability to stand up for truth and what she believes in.
In the last few chapters, Julia's position has crystallized as a role model and voice of reason for Jason. She has gone to college, and he misses her a lot. In Disco, Julia convinces Jason to go to the disco and she instructs him on what to wear and how to get ready. He listens diligently as only a younger sibling listens to advice from an older sibling they have a lot of respect for (I know this dynamic). In January Man (the one at the end), she is home for winter break and to help Jason and their mom move to Cheltenham. She comforts Jason and offers inspiring words to give him hope in this difficult time for Jason. Jason's relationship with Julia solidifies just as their parents' gets rocky (pun somewhat intended), and this is not only unexpected, but also much needed for Jason.
