Michael Hofstadter is the younger brother of Leonard Hofstadter and their sister. He is a tenured law professor at Harvard University. His mother Beverly mentioned him to Howard Wolowitz when she visited ("The Maternal Capacitance"). She describes him as being more successful than Leonard.
In "The Pants Alternative", it is revealed that when Leonard was young he had to give back a ribbon he won for an experiment seeing if Lima beans grew better with classical music when his mother pointed out that his younger brother Michael had already done a Lima beans growing worse with rock ‘n’ roll music experiment.
In "The Spock Resonance", Leonard reveals that Michael was mean to him.
In "The Explosion Implosion", Beverly mentions that of all her children's spouses, Penny is her favorite. This implies that Michael is now married, most likely to the New Jersey judge.
In "The Celebration Reverberation", Michael sends Leonard his Christmas letter and telling about all his accomplishments, including building a hospital in Rwanda. This causes Leonard to spiral about his own lack of accomplishments.