Leo is the most outspoken of the two Stahl brothers. He's the "people person" of the restaurant and loves to talk to the customers. He spent some time on the caravan lines in his teens and has traveled to a few places around the wasteland. It was during this time that he picked up his addiction to jet, although he has managed to conceal his addiction from everyone in town, including his own brother and sister (although it is obvious that Mister Burke and Colin Moriarty know). 
Colin Moriarty writes in his personal computer about Leo Stahl: "Never trust a junkie. That's what I've learned trying to deal with this piece of work. He wanders in here higher than a bird in flight and tried to make it with Nova. She slapped him silly after he didn't want to pay, the good girl. I almost kicked him out myself, but he staggered out of here. That guy may just be the straw I need to break Andy's back."
Leo's Drug Habit: After discovering Leo's drug habit, you have a choice of helping him kick his habit or having him sell you chems. If you choose to help him, he will give you a key to his locker in the Water Treatment Plant and ask that you get rid of all of his drugs. Even if you help him he still stays at his desk in the Water Treatment Plant.
If you confront him over his addiction while he is still in the Water processing plant, you still have the dialogue option of "What are you doing in here?". He will reply the same as before ("Nothing. I'm...I'm not doing anything. Leave me alone!") even though he already knows you know of his habit.
He is one of the few people that will acknowledge Moriarty's death saying "Moriarty thought he was. Not so much anymore I guess, I am surprised Simms never took care of Moriarty himself."
"I can hook you up. A solid chem dealer is hard to find out here in the Wastes, trust me. I know. I can only sell to you when my shift at the Lantern is over. Meet me up at the water treatment plant if you need something."