Trista is a girl that moved to Greg's neighborhood in the final sections of Diary of a Wimpy Kid: The Last Straw. Not much is known about her except that Greg seems to have a crush on her based on the final few pages in the book. It's possible that Trista likes Greg too, because we see her say Hi to him very friendly. As for Rowley, not so much. She appears again in Diary of a Wimpy Kid: Dog Days when a lifeguard pays a lot of attention to her when Rowley and Greg invited her to Rowley's country club but will later appear in book 5 in the same school as Greg.