Little is known about Ronald Reuel. He became the Summer Knight and lived to an old age. He considered four young changelings—Meryl, Fix, Lily, Ace—as friends. He frequently protected them from Lloyd Slate, the Winter Knight. Just before the events of Summer Knight Slate murdered him with the help of the Summer Lady Aurora, who then stole his mantle. The police ruled it an accident.
In the series
In Summer Knight, Mab tried to hire, rather forcefully, Harry Dresden to find the murderer of Ronald Reuel, an artist. She also tells Dresden that something important was stolen, and that he needs to find it. Later, Bob tells Harry that Reuel was the Summer Knight, and what was most likely stolen from him was the power, the Mantle, that is bestowed upon a Sidhe Knight. Bob said that the Sidhe probably don't care about Reuel's murder, they only care about the Power—the Mantle.
While investigating Reuel's apartment, Harry snatched a photo off of Grum that shows Ronald and four young people, standing happily in front of a ride at a Disney park. Grum had been trying to remove it from Ron's apartment. Those four young people turned out to be Changlings—Meryl, Fix, Lily, Ace—that Reuel had been protecting, especially from Lloyd Slate, the Winter Knight. Slate frequently harassed Lily, and now that Reuel is dead, she has no protection and has gone missing. Meryl and Fix asked Harry to find her.
Eventually, Harry discovered that the murderer was Lloyd Slate with Aurora behind it. When Reuel died, the Mantle transferred into Lily, the closest Summer fae at the time of his death. Lily was turned to stone by Aurora, which is why no one could find either Lily or the |mantle. 
Harry reported to Mab on the battlefield in the valley of the Stone Table that Slate is a traitor, and that Aurora stole the Mantle and was behind Reuel's murder, he can get proof if he gets there in time. With the help of the Wee Folk, Aurora is killed and Slate is taken captive by the three Winter Faerie Queens—Mab, Maeve, and Mother Winter. In return for Dresden's help, Mab granted safe passage to the White Council.
In Cold Days, while fighting Fix, Dresden recalls that the last time the Knights fought it wasn't in open combat; Lloyd Slate iced the stairs underneath Ronald Reuel's feet and pushed him down them. There was also a much greater age difference between them.
It's almost certain that Reuel's name is a homage to J. R. R. Tolkien, [Footnote 1] whose given names were "John Ronald Reuel", going by "Ronald". This is emphasized by his job description being "creator of worlds."