A Starfleet lieutenant of the 32nd century who pilots Sylvia Tilly's cadet training flight in 3190 and dies when the shuttle is brought down on the frozen moon Kokytos.
Lieutenant Callum appears in the Discovery episode All Is Possible, set in 3190. Dr Kovich asks Sylvia Tilly to lead a simple team-building exercise for a group of Starfleet Academy cadets who cannot get along, and Callum is assigned to fly the shuttle, with Adira Tal also aboard.
A gamma-ray burst knocks the shuttle off course and it crashes on Kokytos, an icy moon far from the planned route. Callum is killed in the impact, leaving Tilly as the only officer in charge of Adira and the three cadets, Harral, Taahz Gorev and Val Sasha.
His death raises the stakes of what was meant to be a routine outing, and Tilly's handling of the survivors, hunted by the moon's predators, leads directly to her decision to teach at the Academy.