Genghis Khan

Genghis Khan is a member of Maxim Slaughter's "A" List. Slaughter kept a phone number to his friend in his personal organizer. As his personal phone number suggests Genghis Khan deals with killing. Perhaps he is an assassin, or he teaches Slaughter and his men different and efficient ways of killing their enemies, and any who cross them.

Behind the scenes
It's a little odd that Genghis Khan is still alive when the game takes place. However it's possible its just an 'alias' of someone who admires the historical Genghis Khan, or somehow Genghis Khan found immortality, or long life during his reign in the ancient past.