What's that? You thought Titus was a character? True, we do talk about him over in the "Characters" section… Still, though, he's definitely a symbol as well. Consider him a literary jack-of-all trades.
Titus, Sym's imaginary friend, is a floating symbol in that he represents more than one thing. First and foremost, he is a stand-in for Sym's father, who recently died. "I remember the day that Titus arrived in my head […] like some distant cousin who suddenly comes to visit" (2.1), Sym tells us. It's hardly a coincidence that it was the same day as her father's death.
On another level, Sym created Titus as a symbol of romantic love, which she's not yet ready to experience in real life. When Sym tells her friends about her imaginary boyfriend, it doesn't go over very well. "I tried to say that I was happy to stick with imagining for the time being" (1.15), she recalls, "And after that I was the mad girl—sad, frigid, and mad, all three […]" (1.16). Like a good boyfriend, Titus is always there for Sym. But being imaginary and all, he's not too demanding when it comes to sexy times.
Finally, Titus symbolizes the thoughts Sym's most afraid to acknowledge—he's like a part of herself she keeps buried deep inside. When Sym is excited about going to Antarctica, Titus is the one who seems afraid. He's also the one who first notices that Manfred and Sigurd are not who they say they are. Perhaps most importantly, Titus is skeptical of Uncle Victor, whom he refers to sarcastically as "the man who says the Earth's hollow" (17.8). Sym, as herself, can't bear to contemplate the possibility that Victor's off his rocker. But Titus? He totally can.