Throughout Glass Onion, we see a disheveled man whom Miles calls "Derol" whom he claims to be "working through some things," and who has minimal interaction with other characters. However, he does not appear to be involved in any of the intrigue, and Benoit Blanc does not mention him in his summation of the facts of the case.
Of course, in terms of the murder mystery and thus the main plot, Derol may be meant as a rather obvious and humorous red herring. Still, one way or another, Miles still had invited him to his very exclusive private island and let him stay there during his event, so he presumably meant something to Miles. In short, who was Derol and why was he on the island?