Michael Masters was an esteemed electronic and biological engineer before the bombs dropped. He doesn't remember much of the first years after D.C. was destroyed, but he does remember his human life and resents his ghoulish nature (though he does appreciate the gift of longevity). He was extremely intelligent and jovial in his old life, but living as a ghoul has made him very pessimistic and angry at the world. He sees his ghoulness as a karmic curse, because he was attempting to keep America at the forefront of human mutation experiments, breeding super-warriors after being inspired by the works of the German, Russian, and Chinese doctors.
If you complete the Tenpenny Tower quest by convincing the other residents to let the ghouls in, Michael will have a shop and will sell you random guns, ammunition, and other items with a nice discount.
If you choose to kill Roy Phillips after the ghouls move into Tenpenny Tower, even if done silently with no observers, Michael Masters will turn hostile and further trading with him becomes impossible.
pc If you decide to kill all the residents and let the ghouls move in, when you walk into Tenpenny Tower you may see him walk towards the door and then walk out. If you follow him he will walk towards the main exit and disappear when he gets to the potted plants, and you will hear the sound that happens when you go through a grate. If you search Warrington station he will be nowhere to be found. [verified]
Fix: He will be at the station, waiting for the The Lone Wanderer to greet him. Go past the escalator to the metro line and he will greet you. He will then go back to Tenpenny Tower.
Since you can't buy from Lydia Montenegro if you end the Tennpeny Tower quest with good Karma, you can't buy anything for your Tenpenny Tower Suite if you kill Michael Masters before you end the quest. [platforms tag needed]
pcps3xbox360 He may be hostile for no apparent reason upon first meeting him or inside Tenpenny Tower. The only known solution is to load a previous save or kill him. [verified]