By "tragic", I mean that Yuka's human life invokes sympathy from the viewer. When she dies and becomes an Orphnoch, it somewhat makes everything utterly chaotic for her. Occasionally she snaps and kills people despite how she doesn't want to. Then she is attacked by humans later on. Before that, Kaido has never really cared for her when she liked him. To Yuka, hardly anyone doesn't hate or accept her, bar a few such as Keitarou and Kiba. Throughout her life (& death) she had been mostly unhappy. Her final wish was to see a world with no fighting- and tragically and ironically, she does die and presumably go to such a world.
I wouldn't say that everyone brings tragedy. Kusaka's death was no tragedy except for Mari. To many, his death was welcomed. Yes, Mari was "killed", but she came back. The Ryusei members mostly died out, but a few were a**holes anyway. Kaido brought only some comic relief. Keitarou's relationship with Yuka is what makes his part remotely "tragic" at all. Sawada was no real loss. Takumi lives happily ever after (presumably)..............the only character who actually brought some real tragedy too is Kiba. He loses his loved ones by death and betrayal. He later betrays humanity too by accepting his role as an Orphnoch and ultimately dies at the end having redeemed himself.