Bacchus is said to be "less dumpy" then his Greek form, Mr. D. His clothes are not as loud and he seems more organized and thoughtful. As all Roman gods are, he is also more war-like and militaristic. He carries a thyrsus, a staff covered with ivy vines and leaves and topped with a pinecone.
Bacchus appears to Piper McLean in a vision in her knife, holding a silver goblet and gesturing to her. Behind him was a sign that read: Topeka 32.
The demigods head to Kansas on the Argo II, and Percy Jackson, Piper McLean and Jason Grace leave the ship to find the sign Topeka 32. They stumble across Bacchus, whom Percy manages to annoy. Piper finally intervene la before Bacchus changes Percy into a dolphin, and charm speaks him into talking with her. She tells him, hale has to help them with their quest, and he takes it the wrong way. Bacchus states that to decisive help from him they would need to offer him a tribute, which he knew they didn't have so did not tell them what it was. Bacchus, later into the conversation, leaves because he senses danger nearby.
Later into the book, Percy defeats Chrysaor, (son of Medusa and Poseidon), and his army of hybrid dolphins, by scaring them away with Bacchus' name. In the end, they take the enemy ship, with more than 6 million in gold inside and use the Horn of Plenty to fill it with Diet Coke as a tribute to Bacchus.
When they arrive in Rome, Percy, Jason and Piper have to face the giants. Bacchus appears in the middle of the battle, and thanks them for the tribute. He doesn't jump into help though, but instead makes a show out of them fighting the giants and raises them up into the Colloseum. He tells Percy and Jason that if they do well fighting the giants, he would pop in at the end and finish them off.
Percy and Jason take out Otis and are still working on his twin brother, when he disarms both boys.The Argo II blasts him from behind and Bacchus finally comes to help. He taps the giants on the head with his thyrsus, and they both explode into dust.