The portrait on the $50 bill is of Ulysses S. Grant. He was the commanding general of the Union Army and was later elected as the 18th president of the United States.
With the passing of the series of 1928, all U.S. currency was changed to a smaller size. A portrait of Ulysses S. Grant was chosen to appear on the new $50 bill.
Ulysses S. Grant was a prominent war hero, accepting the surrender of Robert E. Lee at Appomattox which ended the Civil War. He had never held a public office before he was elected president, and relied heavily on the opinions of others. The corruption that resulted led to the implication of much of his administration during the Crédit Mobilier scandal.