Don Eppes (Rob Morrow)
- Big Brother Instinct: Don has always been protective of Charlie, even when they weren't that close as kids. As such, when antagonists in particular target his brother, he goes on the offensive.
- Brains and Brawn: He's the badass FBI agent to his math professor brother, what'd you expect?
- Chick Magnet: Seriously, the guy has had at least five girlfriends.
- Fair Cop: FBI and quite a looker.
- Knight Templar Big Brother: When one guy went after Charlie, Don walked right into his office and told him right to his face that he would take him down hard for going after his brother.
- Papa Wolf: Threatening his family, team or associates is a good way to make Don pissed at you. Exploited in an episode. The Russian mob intentionally went after his brother and team as an elaborate distraction from their scam.
- Sibling Team: The premise of the show is he's this with Charlie.
- The Stoic: Don has a tendency to be a stone wall with his emotions. His girlfriend actually lampshades him doing this.
- Team Dad: To be expected as the head of his unit. He looks out for them.
- The Un-Favourite: Not intentionally, but due to being a math child prodigy, Charlie required a lot more of their parents' attention when they grew up. Alan has some guilt about it and Don initially has some resentment, but they get over it.
Charlie Eppes (David Krumholtz)
- Absent-Minded Professor: Not to the extent of Larry, but he has his moments. Downplayed otherwise since he connects well with his students.
- Adorkable: Whenever he gets into his math teachings he can be this. He never lost his passion for math.
- Annoying Younger Sibling: He is younger than Don by 5 years. While he didn't pester him, Charlie's advanced mind often made things awkward for the both of them growing up.
- Badass Bookworm: Post-Character Development. He has become better able to deal with hostile situations and learns how to shoot quite well in fact.
- Brainy Brunette: Dark brown curly hair? Check. Mathematical genius, check.
- Defective Detective: Charlie, a mathematical genius who uses his talents to help his FBI agent brother solve crimes, has emotional difficulties to the point that he locks himself in the garage to work on unsolvable problems when he can't deal with life. However, he overcomes this early on.
- Gentleman and a Scholar: He's quite approachable and a gentleman, no doubt to being raised well by his parents.
- Good with Numbers: Up to Eleven. He went to univerisity at 13 and mathematics remains to be his strongest passion.
- Insufferable Genius: He can be this at times, especially when confronted by a rival.
- Jewish and Nerdy: Downplayed. While quite nerdy, he doesn't think much on his Jewish background. Becomes more noticable when Don takes a greater interest in Judaism in later seasons.
- Nice Jewish Boy: He is still Jewish and quite a nice young man.
- Non-Action Guy: While he does later learn to shoot firearms, he is still primarily best when at the brains.
- The Professor: Professor Charles Eppes thank you very much.
- Quirky Curls: Amita certainly likes them.
- The Scully: Reacts this way to psychics in constrast to the more open minded Larry.
- The Smart Guy: To pretty much the whole team in general.
- Smart People Play Chess: To the point that the only chance that most people have at beating him is for two people to play him at once while also distracting him.
- The Storyteller: Charlie teaches math with stories.
- The Strategist: Charlie would probably qualify with his ability to mathematically predict the moves of evildoers.
Larry Fleinhardt (Peter MacNicol)
- Absent-Minded Professor: Oh yes. He's a very quirky person and has a tendency to be philosophical, which makes him amusing to the eyes of his students.
- Berserk Button: Somewhat downplayed; The episode "Atomic No.33" has the team deal with a cult; their regards to faith-healing and disregard to medicine and science makes him angry, albeit in a subdued manner. Still, this is probably one of the angriest we've seen him.
- Bunny-Ears Lawyer: A very bright and valuable man; he even went into space once. However, he also sold his home out of not wanting to constrain himself and once had a diet of only white foods, among other quirks, such as staying in bathtubs for a long time (which is related to the home thing).
- Cloud Cuckoolander: He comes off as this at times, due to his selling his home, that time he ate only white foods, long periods in bathtubs and his eccentric philsophical nature in general. Charlie and the others help keep him down to earth.
- The Gambling Addict: A former one, and his experience comes in handy when a prior colleague of his does the same scheme they did when they were younger — card-counting.
- Gentleman and a Scholar: While he be may an eccentric, he is still quite the gentleman and a very knowledgable physicist.
- The Lancer: To Charlie when it comes to the folks of Cal Sci.
- Like a Son to Me: It becomes increasingly evident that Larry views Charlie as this and when Larry went off on a sabbatical, he entrusted his work to Charlie in a way a father would to his son. Charlie in return very much looks up to Larry.
- Opposites Attract: Philosophical and dorky Larry is mutually attracted and has a relationship with practical and analytical profiler, Megan Reeves. Very much lampshaded how odd the couple are.
- Parental Issues: Not to the extent of Megan, but he had problems with his father wanting him to become an artist.
- Put On A Space Shuttle/The Space Shuttle Came Back: Left in "Brutus" for doing research on the ISS. Landed safely in the "The Art of Reckoning".
- The Professor: A physics professor at Cal Sci and he was Charlie's professor back when he was a student.
- Sesquipedalian Loquaciousness: Part of what makes him come off as an eccentric professor, remiscent of the stereotypical kind.
- Smart People Play Chess: He often plays chess with Alan He becomes much better at it when he returns from his sabbatical.
- The Spock: Downplayed. While Larry can show joy and sadness, he is often in a state of logical calmness and relies on logic and rationale. He's also not that good with dealing with passion and emotional uncertainity, which was examined in his budding relationship with Megan.
Alan Eppes (Judd Hirsch)
- Good Parents: He does an admirable job in raising his son to be honest and good men and he still looks out for them with advice and assistance. No wonder Charlie and Don turned up as well as they did.
- I Want Grandkids: Alan takes the proactive approach, giving solid relationship advice to Don and Charlie.
- Team Dad: Not just to his sons, but to their co-workers as well, serving as an impromptu father figure. He also serves something of an older brother figure to Charlie's mentor Larry.Alan: I'm going to give you the same advice I give to all the geniuses I know.Larry: And that is?Alan: Don't be an idiot.Larry: Ah. (nodding)
Amita Ramanujan (Navi Rawat)
- Arranged Marriage: Amita was in one and knew the guy as a good friend since she was young. Her parents invited him back to try and win Amita over and she does greet him warmly. When Charlie voices his concerns though, she explains that he is gay, something Charlie and her parents weren't aware of. Not only does Amita realize how awkward it made Charlie feel, but what her parents attempted to do.
- Gamer Chick: As revealed in the episode "Primacy". Even when she was supposed to lose, her gaming reflexes kicked in and she won anyway.
- Parental Abandonment: Not quite, but Amita's parents are always too busy to visit her.
David Sinclair (Alimi Ballard)
- Bald Black Leader Guy: Although not technically a leader, he is Don's second-in-command.
- Dark and Troubled Past: He grew in a rough neighborhood with troubled family members. He once described having to dodge gangs to graudate high school.
- Heterosexual Life-Partners: With Colby. The two are often seen side by side one another when investigating and David took it the hardest with Colby supposedly being a double agent, later revealed to be a triple agent.
- The Lancer: He's Don's right-hand man and the one people listen to when Don's not around. Fittingly, he takes Don's position when Don ends up moving on into a different role.
Colby Granger (Dylan Bruno)
- Cold Sniper: Of course, all the FBI agents have to be this to a degree, but somehow Colby ends up having to shoot criminals more often than the others.
- Deadpan Snarker" Colby does has his moments.
- Fake Defector: Colby turns out to have been a triple agent for several years. This causes a fair bit of drama and angst, since the rest of the cast find out about the "defector" part well before they find out about the "fake".
- Heterosexual Life-Partners: With David.
- Hidden Depths: Is fluent in Spanish, which comes in handy when the team investigates the case of missing Spanish girls.
Megan Reeves (Diane Farr)
- Action Girl: The most frequent female team member we see in the action and she isn't afraid to kick ass when need to.
- Deadpan Snarker: Likes to snark a lot, especially with Colby.
- Former Teen Rebel: As revealed when she was kidnapped, she used to be a fair bit wild when she was younger. It's implied this played a role in her choice to leave the FBI to counsel women in prison.
- Nerds Are Sexy: Dates Larry, to everyone's surprise.
- Opposites Attract: Philosophical and dorky Larry is mutually attracted and has a relationship with practical and analytical profiler, Megan. Very much lampshaded how odd the couple are.
- Parental Issues: Has a serious problem with her dad that resurfaces every now and again.
- Probably why she was drawn in by someone like Larry, outside of him having some problems (albeit not to the same extent) with his own father.
- Put on a Bus: She ends up leaving the FBI to become a counselor to women in prison at the end of Season 4.
- The Profiler: She is the one who psychoanalyzes their suspects and trying to figure out their motives.
- Why Couldn't You Be Different?: According to her, she was her father's "last attempt to get a son."
Liz Warner (Aya Sumika)
Nikki Betancourt (Sophina Brown)
- Action Girl: Takes Megan's role of this starting in Season 5.
- Deadpan Snarker: Often manages to outsnark Colby.
Terry Lake (Sabrina Lloyd)
Ian Edgerton (Lou Diamond Phillips)
- Anti-Hero: Out of the FBI, Ian's the one most willing to use extreme means to complete his task, most apparent during "Ultimatum", and shows no hesitation in killing. He's still an overall a good guy, though.
- Cold Sniper: You bet it! It sort of comes with being the fifth?.. Fourth?.. Third best sniper in the country. He lives up to the reputation, too. If he's in an episode, he'll usually outshine the agents by being the one to take out the criminal of the week.
Robin Brooks (Michelle Nolden)
- Heroes Want Redheads: Don's on-again-off-again girlfriend. And fiancee, by the end of the series.