The man described here is a gold prospector named Hermann Kermit Warm.
The description comes up when Eli and Charlie Sisters are discussing the man they are supposed to kill next. The assignment of killing Warm comes from the man the two brothers work for, a crime boss known as The Commodore. Charlie is explaining to Eli that The Commodore has chosen Charlie to be the lead man for this upcoming assignment. Charlie then takes out a letter sent to him from Morris, The Commodore's scout who has been studying Warm for a time. A larger description can be found in the letter:
“Have studied Warm for many days, and can offer the following in respects to his habits and character. He is solitary in nature but spends long hours in the San Francisco saloons, passing time reading his science and mathematics books or making drawings in their margins. He hauls these tomes around with a strap like a schoolboy, for which he is mocked. He is small in stature, which adds to this comedy, but beware he will not be teased about his size. I have seen him fight several times, and though he typically loses, I do not think any of his opponents would wish to fight him again. He is not above biting, for example. He is bald-headed, with a wild red beard, long, gangly arms, and the protruded belly of a pregnant woman."