Tris taking in an orphaned Glaki, planing on bringing her to Rosethorn and Lark when they get back home.
For that matter, Tris taking in Chime, a strange magical creation after protecting it from it's frightened creator.
Keth giving Tris a neckles made from Chimes flames, then telling her that he has no intention of taking her back.
Keth: I think she (Chime) belongs with you.
Briar's reasoning behind the date he chose for his birthday at the end of Briar's Book/The Healing In The Vine. "The girls would be vexed at having so little time to prepare, but they would adjust. They would know, as he did, that his life began when Rosethorn had invited him into her world."
And before that, when Rosethorn was dying, she said to Daja "My boy. You three girls—look after Briar. When I'm gone." She doesn't stay dead, but it doubles as a Tear Jerker and a moment of heartwarming.
The last lines of the last book. Briar explains the little mind palace created for the four in his dreams.
Briar: It's us.
All: It's always us.
In The Will Of The Empress, when Sandry first gets kidnapped, Briar and Tris, despite not getting along with her so well, instantly drop everything and go to help as soon as they find out what's happened.