Billy Dilley appears to be something of an idiot savant. He is socially inept and a nuisance to those around him. He does not seem to catch on very easily to social cues and is unaware that he is causing discomfort to those around him. Despite this, he cares very deeply for his friends Zeke and Marsha, most likely because he never had any friends back home, and always tries to look for a silver lining in any situation, showing bouts of optimism.
Billy displays a strong interest in the sciences and usually utters the word "fascinating" when something interests him. This will sometimes cloud his judgement as he openly ignored Judy when he became more interested in a common earthworm. He ultimately learns his lesson and extends his friend list with Judy, whom he seems to have a slight crush on.
Billy at times can be his own worst enemy. He has shown to be highly selfish when it comes to things that obtain his interest and will go out of his way to push people so that he can get what he wants. Conversely, he has shown to be very concerned with others such as his pet rat Anaximander whom he was afraid was depressed and did everything he could to make him feel better.
Billy Dilley is a short chubby Caucasian boy. His checks are so big, they slightly cover his mouth and he has freckles. He has large eyes and a small nub of a nose and his head is topped off with a giant orange afro which always seems change density depending on the situation or gag.
He wears a tight turquoise shirt and light cyan pants along with blue sneakers and white gloves. Barely sitting behind his afro is a hat with a light green brim and a blue and white base with a noticeable "B" on the front; representing Billy.
Billy Dilley is a scientific genius and will use whatever he has at his disposal to build or create so as to solve any problem. Despite never having been to Subterranea-Tania prior to the series, he is well informed enough to identify certain species and flora and fauna that reside underground based purely on his own personal research.
Billy seems to look up to Zeke and is always trying to get him to get up and do things with him. He is unaware of the fact that Zeke lacks any real interest in what he does, but that does not stop him from trying. The last thing Billy wants to do is make Zeke angry or upset and tries to put his needs ahead of his own. When he thought Zeke was sick, he was willing to let himself be infected so that he can spend time with him. Billy seems to spend more time with Zeke than he does with Marsha, implying that he considers him his best friend.
Billy sees Marsha as a good personal friend of his. In the first half of the series, his antics seemed to get on Marsha's nerves. By the second half of the series, Marsha's antics seemed to slightly bother Billy. His relationship with her is a lot warmer than with Zeke's, as he only seems to be slightly a nuisance to her. Billy puts Marsha's feelings ahead of his own as he was afraid of making her angry. However, he learns that he can be open and honest with her at any time and the two have since, trusted one another.
Billy adores his pet rat Anaximander. He always relies on him, despite the fact that he does not seem to care about anything. Billy found him in a dumpster by a factory and nursed him back to health. Since then he has been by his side. He does not seem to realize that Anaximander has his own plans and interests, but they do seem to keep an eye on one another, though Anaximander did seem to abandon Billy and his friends by the series' end.
Billy immediately found Judy fascinating upon meeting her. However, he proceeded to ignore her once he found a common earthworm. Afterwards, he began to dismiss her almost immediately. He eventually came back around to noticing her upon realizing how much she cared about him and it is heavily implied that he starts gaining a crush on her. He became deeply hurt when she claimed that he was never her friend, but after saving her and Subterranea-Tania, she suggested he stay.
Billy believes that Count Wretcher is an interesting person. When Wretcher showed an interest in his Cheeserator, he took it as a sign of mutual acceptance. He is unaware of the fact that Wretcher simply wants to use him to get back to the surface world and cover it all in sludge. Despite his sinister intentions, Billy views him as a friend and wants to help him. He became deeply hurt when he claimed that he was never his friend, but after saving him and Subterranea-Tania, they seemed to be on better terms, at least at the moment.
Billy appears to admire the Hag Witch, mostly because he views her as being a chemist rather than a witchcraft practitioner. He desperately wants to serve as her apprentice and impress her any which way he can. He became deeply hurt when she claimed that he was never her friend, but after saving her and Subterranea-Tania, they seemed to be on better terms.
- He has an allergy to petunias.
- He appears to have cacophilia, an extreme love of, what he perceives to be, ugly things.
- He has poison oak rash on his legs that he has "been cultivating since the third grade".
- Jessica McKenna was originally going to voice the character Billy Dilley, but was replaced by creator Aaron Springer for unknown reasons.
- He is the only character out of the main four, alongside Zeke, Marsha and Anaximander, to appear in all episodes of the show.