Doctor Lesko

 created the Fire Ants that carried off or killed almost everyone in Grayditch. He was experimenting to return giant ants to their normal size through genetical manipulation and accidentally created the fire ants.

 has a shack by the diner which requires Average Lock-picking Skill to enter. He can be found deep underground in the Marigold Station. You will receive a quest from him that offers a perk as a reward. (It has been reported that he has been found dead by some players -- for now I would suggest not waiting too long to complete the "Those!" quest until someone figures out the exact parameters for why this is). Lesko can sometimes be killed by fire ants if you leave both doors to his office open, close them after speaking to him to ensure he is not killed before you have completed his quest.

Apperances
 only appears in Fallout 3.

Related quests

 * Those!