Many villains appear in Fullmetal Alchemist: Brotherhood, but one of the most influential -- second only to Father himself -- is Pride the homunculus.
Pride is exactly that: Father's discarded arrogance and feelings of superiority made flesh. However, Pride is also one of the most deceitful villains, aside from Envy, and his true nature is revealed only one bit at a time. He casts a long shadow (almost literally) across the entire story.
Pride Was the First of His Kind
Pride is acknowledged as being the firstborn homunculus, because the other six know him as their big brother. This makes Pride around 300 years old, and he certainly does have the confidence of a firstborn. Pride routinely looks down on humans, but not in a taunting manner like Envy. Instead, Pride is arrogant about his superiority, viewing human beings as short-lived, powerless ants that are beneath his notice (a view that Father shares, but with less obvious scorn). This attitude is apparent in battle when Pride speaks confidently about his ability to defeat any foe and control any situation, and he often proves himself correct... But not always. He sometimes takes his alchemist foes too lightly, and Edward, Alphonse and Hohenheim all got the better of Pride when he let his guard down.
Pride's real body is a large, shadowy mass that can easily alter its shape and size at will, loosely related to Envy's own shapeshifting abilities. However, these shadows don't roam freely; they need a container. Pride has two such containers: his hollow "Selim Bradley" body, and the circular tunnel that Sloth dug around the entire perimeter of Amestris. So long as Pride remains within either one, he is safe, but he would evaporate in the absence of those containers.
Pride's shadowy body is covered in mouths that can eat other beings and absorb their powers, and he also has many eyes that allow him to spy on anyone, anywhere. On the offensive, Pride can elongate parts of his body into hard spears that can stab his enemies (like Lust), or he can form arms that grab or lift objects or people. These arms and spears can move and elongate quickly, taking enemies by surprise. And unlike his six siblings, Pride needs the right lighting to fight; in absolute darkness or overwhelming light, there are no shadows, and his body must retreat inside one of its containers. The exposed parts of his body dissolve away. For this reason, Pride likes to fight during the afternoon or evening, when there are sufficient levels of both light and dark.
Pride's Role in Fullmetal Alchemist
Pride appeared fairly late in the story of Fullmetal Alchemist, but he had some influence even before he appeared in person. He first showed his face in his Selim Bradley disguise, fooling the other characters and first-time viewers alike, acting like a polite and enthusiastic boy who adored Edward Elric, the oh-so-cool fullmetal alchemist. He was supposedly the son of King Bradley, but in reality, they were both homunculi, and therefore brothers. Pride also made a cameo during the Fort Briggs arc of the story since his tunnel-container passed directly underneath Briggs. Once Sloth exposed himself and was caught, Briggs soldiers were sent to check out the tunnel, soon running afoul of Pride.
Lieutenant Riza Hawkeye visited the Bradley family and after speaking with Mrs. Bradley about Selim, she realized that Selim and King Bradly were not who -- or what -- they appeared to be. Pride confronted Riza that evening in the street, coercing her into keeping quiet about her revelation. As told, Riza didn't openly tell anyone about Pride/Selim Bradley's true nature, but she did covertly slip the message to Roy Mustang, who got the message without tipping off Pride.
It was Pride who acted as one of Father's last guardians during the Promised Day, fighting even after Envy, Sloth and Gluttony succumbed (in fact, it was Pride who ate Gluttony to steal his powers). Pride confronted Edward's party, and he and Solf J. Kimblee fought Alphonse Elric. Pride had been contained inside an earth dome to shut off his powers, but sent a morse code message to Kimblee, who freed him. Alphonse managed to escape Pride, and Pride continued to fight on Father's behalf, including overseeing the sacrifice of the chosen five sacrifices for Father's plan (Ed, Al, Izumi Curtis, Roy Mustang and Hohenheim).
Then, Pride made his last stand against Ed, and his Selim Bradley body started to come apart. Ed finished the job when he converted himself into a human Philosopher's Stone and fought Pride's soul directly, and once he beat Pride into submission, he witnessed Pride's true nature: a frightened young boy. Then Pride's true physical form appeared, a tiny fetus-like entity with a dot on its forehead. Pride fell at last.
In an epilogue scene, Pride was reborn as a younger Selim Bradley, still with that mark on his forehead but no memories of his past life as Father's top lieutenant. It is left ambiguous as to what Pride's future might be. But for now, at least, Amestris was free of the shadow of Pride's fearsome power.