Dr. Lawrence Fleinhardt is a professor of physics at CalSci (a university based on Caltech and located in Los Angeles in the Numb3rs universe) and best friend and colleague of Charlie Eppes. He is portrayed as a brilliant theoretical physicist and cosmologist, who researches the cosmic microwave background, supersymmetry, string theory, 11-dimensional supergravity theory, doubly-special relativity, black holes, and gravitational waves, using LIGO to check predictions on quantum corrections.

He may have even found a way to express Calabi-Yau manifolds in a way that goes beyond a non-vanishing harmonic spinor and, independent of Charlie, published a work of genius entitled Zero Point Energy and Quantum Cosmology, which could provide insight into the cosmological constant problem (episode 3x4,The Mole). Larry has also worked on the photoelectric properties of Ly-alpha emitters in a QZ2 plane universe. Dr. Fleinhardt holds the Walter T. Merrick Chair in Theoretical Physics.Larry Fleinhardt has always been fascinated with the stars. He had his first telescope at age three, was the president of the rocket club in the fifth grade, and worked as a docent at his local planetarium while in junior high school. Once a remarkable student comparable to Charlie, Larry graduated from college at the early age of 19.

However, Fleinhardt had an addiction with card counting during his years as an undergraduate, which he feels ashamed about and has compared his discomfort during field research at a casino as being at the event horizon of a black hole; this was discussed in Double Down, an episode that focused on a gambling ring. He wanted to prove the power of mathematics. Dr. Fleinhardt has been teaching for 20 years and Eppes attended his classes at Princeton University, first taking his quantum physics class at the age of 13. Charlie's father, Alan Eppes, said that all the family heard about that first year was Professor Fleinhardt. Dr. Fleinhardt has worked on building a space telescope for DARPA in the past, but it was weaponized against his wishes as part of the star wars missile defense project.

The only thing he remembers of his mother is her warm smile and tweezed eyebrows. He rarely has spoken of his father, a painter who wished his son had seen the world the way he did and was disappointed by his son's path; Fleinhardt expressed his sorrow with a memory he does not gladly inhabit, stating it is a father's folly to impose their will on their son. Consequently, Larry has an exhaustive knowledge of art and attempted a career as a painter, but failed as he was required by his father to study the great works of the masters before he could create an original work.

Larry is quite awkward in social situations; his students have described him as "boring and intellectually inaccessible". Some of his irregular views have made Charlie wonder, "What flavor of crazy Kool-Aid do they make you drink before you join the physics department?" When Larry suggested a neural imbalance may be the cause of his lack of dreaming in "Hot Shot", Charlie thought the notion would explain a lot about his friend. Usually, Larry seems relaxed, but, at times, he can be a bit neurotic, or "Fleinhardt". An absent-minded professor, he has forgotten whether he was entering or leaving the CalSci library.

The "Doc" even had his own Eureka moment in "All's Fair", wherein he applied fluid mechanics to the analysis of footprint slabs he submerged in a jacuzzi only to determine, from the relative displacement of water by each indentation, that the suspect to a murder had a short right leg — a discovery that discounted a particular individual from guilt — and ran to the streets dripping wet in a robe, screaming that it was Archimedes all over again, only to forget where he put his clothes the next minute. Larry is disorganized like Charlie and claims that there is symmetry to his chaos. On a similar note, his antics have led him to eat purely white food at times to maintain supersymmetry, a focus in his research.

Apparently, as mentioned in "Backscatter", Fleinhardt is awful when it comes to geography — he called to ask if the string theory convention he was already at was in St. Louis or Cleveland, when it was actually in Minneapolis. A perpetual preoccupation with celestial phenomena often has him spouting cosmic metaphors. His character is not unlike that of former acquaintance Richard Feynman, who is noted for being both a genius and an eccentric.

Larry frequently wears casual patterned shirts and drove a 1944 Volvo until he acquired a 1931 black Model A Ford that he cherishes. Fleinhardt owned a personally restored 1877 Victorian home which he sold in "Bones of Contention". The character evinces a distaste for number theory and cell phones, but enjoys hiking. Also akin to Feynman, he percusses, but never for pleasure — he drums away problems. Both he and Charlie are members of the North American Sundial Society, and they have worked together on various contests for the physics department, including a paper airplane contest. Larry has a Zen rock garden and has constructed his own Heron's fountain. He likes to "ponder the koi pond" in front of Charlie's home, with each fish named by Eppes.