Diesel 10 is an evil, conniving, bullying, and psychotic monster. He absolutely despises steam engines, and would call them taunting names like "puffball" and "teapot". He is Sodor's strongest but not quite largest Diesel, his power being enforced by his slightly malfunctioning claw, Pinchy. He can use this deftly, and is even able to use it to make a sculpture. All engines, steam and diesel alike, fear and respect him.
Despite his aggressiveness, he will help on occasion since Sir Topham Hatt brought him back, but only when it suits his personal interest. He can have a caring side, but he very rarely shows it.