The leader of the Time Lords at the time of the war between the Megallan Empire and the Telians was a "grim-faced" Time Lord. He had green skin, prominent cheekbones and a drooping moustache. He and his two closest advisors were significantly taller than humans, towering over even the Space Controller, a fellow Time Lord.
After the Time Lords' computers predicted that the explosions detected in the Zeta Four Sector might lead to Zeta Cancri going supernova and the destruction of the entire galaxy (including Gallifrey), the leader and his two advisors summoned the Space Controller of Gallifrey High Command, as well as his agent K9 Mark I, to the Control Centre, where they briefed K9 on the situation. They sent him to the Zeta Four Sector to investigate a distress signal picked up by the Gallifreyan systems, and which was the primary piece of evidence pointing at a manmade origin of the explosions, versus simple catastrophe theory. (PROSE: K9 and the Zeta Rescue)
Behind the scenes
The identity of this Lord President is not given in K9 and the Zeta Rescue. As The Adventures of K9 features K9 Mark I after The Invasion of Time, he could potentially be an incarnation of Borusa, placing between John Arnatt's (as seen in The Invasion of Time) and Philip Latham's (as seen in The Five Doctors). However, although The Five Doctors postdates K9 and the Zeta Rescue in real-world release order, it is entirely possible that the book takes place after Borusa's fall from grace in-universe, and the "grim-faced" leader is one of his successors, such as Niroc.