Doctor Nefario has been in the evil scientist business for a very long time, and it shows.  With an incredibly inventive mind and a knack for problem solving, he is still occasionally distracted by the rigors of old age, which result in his often misinterpreting Gru's requests for new weapons or devices for his despicable plots and missions...with occasional comedic effect.  Doctor Nefario is completely and utterly dedicated to a life of evil, and is generally not comfortable around children.  He spends most of his time down in the depths of Gru's secret lair, usually in the laboratory and machine shops...creating various concoctions, weapons and machinery...and fixing stuff broken by Gru, the minions and the girls.

Above: Dr. Nefario in his younger years, demonstrating his many inventions (including the freeze ray) on the exhibitor floor at Villain-Con in 1968.  Gru and his mother look on.  Is this the start of a long-standing and villainous friendship?
 

Dr. Nefario routinely wears a white lab coat, black rubber gloves (with the Gru logo on them), a pair of goggles, red and yellow plaid pants, a yellow shirt and black boots.  He has a substantial double chin, a sharp hook nose, a pot belly, large and long ears, and a thin, wispy tuft of white hair atop his head.