What is the name of Gru's assistant who invents gadgets?

Gru’s assistant who invents gadgets is Dr. Nefario.

Dr. Nefario is Gru’s elderly inventor and partner-in-crime in the Despicable Me franchise. Working from Gru’s secret underground laboratory, he designs the specialized weapons and machines that support Gru’s schemes, including devices such as the Shrink Ray and Insta-Freeze Gun.

Although Gru is the mastermind behind the plans, Nefario supplies much of the technical ingenuity. His inventions help Gru attempt spectacular crimes, including the plan to steal the Moon in the first film. Nefario is also closely involved with the Minions and serves as one of the few people who understands Gru’s villainous operation from the inside.

A common mix-up is choosing Dr. Evil, Dr. Wily, or Dr. Doofenshmirtz because each is a famous fictional scientist or villain. They belong to other franchises. Dr. Nefario is specifically associated with Gru, the Minions, and the Despicable Me films, and he is voiced by Russell Brand in the earlier movies.