First Appearance: Naked in Death
He is technically Roarke's butler. However, he used to go by the name Basil Kolchek back in Ireland in the 2030s; he had a daughter named Marlena. During the series' fictional "Urban Wars", a time of urban unrest, Summerset served as a medical aide, also working with the title doctor from Origin in Death.
When Roarke was ten, he met Summerset and Marlena, age eight. They ran cons together; shortly after, Summerset found Roarke half-beaten to death in an alley (by Patrick Roarke); he saved him and nursed him back to health. Roarke mentions (somewhat conflictingly) later that he thought Summerset was a mark and tried to steal from him, but as Roarke said, Summerset "dissuaded him of that notion".
Patrick Roarke turned up shortly later dead in an alley, stabbed in the chest and robbed of all money and valuables - supposedly a victim of a mugging. It was a surprisingly innocuous death for a very dangerous man, and Eve realizes in Portrait in Death that it is Summerset who in fact killed Patrick. Summerset says that he did it because he had "children to protect." Roarke does not know as of the most recent book and likely will not find out.
When Roarke was sixteen, he was running a small gambling ring; his competitors wanted him gone. They figured that the best way to deliver the message was through Marlena, Summerset's fourteen-year old daughter. The six men raped, murdered, and mutilated Marlena, then dumped her body on their doorstep. In revenge, Roarke slowly hunted those six men over the next decade, the basis for the plot in Vengeance in Death. When he comes to the U.S., he brings Summerset with him, asking Summerset to become his home's caretaker. He thinks of Summerset as a father.
In that same book, Eve realizes why Summerset does not like her: she says to him that he has always wanted someone like Marlena for Roarke, a sweet and innocent young girl - something Eve has never been. Summerset had not realized the basis of his dislike, and is guilt-stricken over it. In the end of Vengeance, Eve takes a hit meant for Summerset.
The adversarial relationship between Eve and Summerset is a matter of course: she often relieves her work-related stress by insulting him; she throws her jacket on a post because she knows it annoys him, and because he expects her to do so. In Conspiracy in Death, it becomes clear that Summerset is merely fulfilling the role of adversary, but has genuine concern for Eve's well-being. The heart "she didn't know he had" squeezes in empathy for her situation; he tells her that she is late for dinner simply to put the thought of food into her mind (he knows she would have forgotten otherwise); and when a pallid and depressed Eve, soaked from walking in the snow, gives a lethargic apology to one of his particularly withering comments, he goes to Roarke and says, "I insulted her and she...apologized. Something must be done."
On several occasions, the two have had meaningful conversations over their only common interest, Roarke. In Innocent in Death, Summerset fully supports Eve over Roarke's ex-lover, telling Eve that even though he doesn't like her, Eve is the one for Roarke because she genuinely loves him.
Summerset is also very fond of Mavis and tends to cater to whatever she wants on her visits; he treats her very affectionately. Ironically, this may be some indication that Mavis was like Marlena. He gives Mavis away at her wedding, and loans her his own wife's ring, a further indication of a father-like feeling for Mavis.