Leslie is a freelance trader working for United Leon Space Enterprises. He introduces the player to purchasing ships, taking contracts, and picking up commodities.
The player first encounters Leslie Garbut in the game's tutorial. He notices the player pop into the Leo Sector, and realises they are part of the Cassandra mission.
Leslie guides the player to Lago Gas Port, where he instructs you to trade in your ship for a freighter. Leslie puts in a good word with his boss at United Leon Space Enterprises, awarding the player with a license to accept contracts from them.
Leslie then gets you to select a contract, take the commodities from the trading terminal, and deliver the cargo to its destination.
Leslie emails you with some more information, asking that you reply to let him know you got the message okay.