My favorite Tiana moment in history of the Keeper books will always be the moment in Nightfall where Biana stops and repaints the painting of Tam. Painting takes time, right? And even though her brother, who she hadn't seen in months, might possibly be dying and currently in a coma (pretty sure it was one of the two; maybe both), she takes the time to repaint the picture of Tam because it didn't look like him.
And Tam's reaction was described as the biggest smile Sophie had ever seen in wear. Tell me, when as Tam smiled in the books? Barely. When he does smile, it's more of a smirk. And his smile after seeing the repainted picture of him was said to be huge. I remember in the book, there was a full paragraph of Sophie describing the way Tam reacted to Biana redrawing the picture.
I have a lot to say on this moment. It's my favorite interaction of them in the entire series. This was the moment that I really started to ship them. <3