Two-Back has the appearance of a mutilated humanoid with its lower front fused into the lower back of a large monstrous creature. The appearance suggests the two mutilated creatures are participating in a sexual act. The larger humanoid's front arms are stubs that end in horrific needle-like protrusions, while the smaller/"back" humanoid has stubs as both its arms and legs, also having needle-like limbs.
It attacks by spitting acid, or ramming into Travis Grady. It may also grab him, which is quite powerful as it spews acid on Travis's face when it does this.
The Two-Back represents Travis's sexual deprivation as a lone trucker. Among the other Shakespeare references in the game, Othello is also referenced. In the play, Iago describes Desdemona's sexual relationship with Othello by stating that they are "making the beast with two backs". This is a reference to intercourse, supposedly fusing two people into a single beast, in this case, literally.