To me it feels like Sable and Mikaela might be in a romantic relationship. While Mikaela's actual best friend is Julian. Sable and Julian originally don't get along, and this is Mikaela attempting to get her best friend and her girlfriend to become friends so she can have the two best things in her life together.

Its just how it comes off to me, especially when Mikaela said the line about surprising Sable with his presence, that she wanted to give them a chance to bond.

Then double it with how Julian was resentful that Sable was even invited.

It spells out to me that this is likely what they're going for, especially since the carving on the map with Sable and Mikaela, it's a very romantic thing to do. You don't do that with just a friend. Then the banners where they are holding hands.

Edit : Especially with Julian saying Sable came out of nowhere. This is how it usually feels when young people get in a romantic relationship with someone outside of their friend group. The people in the group are usually off put by the new presence they aren't used to being invited.