Luo Ji (罗辑) is an astronomer and sociologist. Given the initial axioms of Dark Forest theory by Ye Wenjie. He is his own Wallbreaker when he concludes that the axioms result in the Dark Forest state of the universe, and thus the only strategic option against the Trisolarians is mutually assured destruction through broadcast of their interstellar coordinates.

He is initially a self-centered and unambitious partygoer, but his life transforms as he develops the "Dark Forest" theory. This theory leads him to create a deterrent strategy by threatening Trisolaris with mutually assured destruction if they invade Earth, making him a crucial, if reluctant, hero who eventually becomes the first "Swordholder".