Ba was an Arix. His species' life expectancy was twenty-four hours. His eyes were green and his favourite colour was blue.
Ba and his mother were abandoned on Earth when their spaceship accidentally entered the TARDIS right before it dematerialised. Though the Fifth Doctor and Peri returned the next day, for Ba decades had passed. Because Ba's speech was so fast compared to the Doctor's, he found it easier to use telepathy to communicate. With the Doctor's help, he found his spaceship and the rest of his people. (PROSE: A Life in the Day)