Selim's role is very minor in the 2003 anime series. He is merely the Führer's timid biological child and appears late in the series.
In the manga and 2009 anime series;
Selim is the Führer's sprightly son. He frequently asks his father to tell him stories from the battlefront and interesting anecdotes from his everyday events. Selim professes a desire to become a State Alchemist so as to be more useful to the Führer and is a self-professed fan of Edward Elric.
SpoilerIt is later revealed that Selim is not Bradley's biological son, but rather adopted from a distant relative and also the Homunculus known as Pride.
Pride is the first Homunculus that Father created. He is the most powerful of the Homunculi and seemingly the highest ranked, as he is seen giving orders to Sloth and Envy. Thus, he is generally respected by the others. He is extremely cruel, heartlessly ruthless and extremely devoted to Father's cause. He refers to his enemies as "mister (plus a nickname)" in mocking respect and often makes big smiles and casual talks before attacking. He has also proven himself to be quite clever and resourceful.