During the banshee banter, Cassie remarks that she'll be "going out with it" (p64). This identifies Wally Rosenquist as one of the 'sidh'. From various sources, we learn the sidh were widely perceived as fallen angels. The common folk referred to sidh euphemistically as Gray Neighbors, Good Neighbors, and other innocuous names designed to avoid provoking these entities.
To reinforce this idea, note that Rosenquist as theatrical money-man is repeatedly called an angel:
"I've got this angel" p55
"India's got an angel" p60
"How long has India had this angel?" p61
From behind Ebony, Tabbi murmured, "I thought she was bringing the angel." p62
"Let's have a real standing 'O' for a real angel--Mr. Wallace Rosenquist!" p66
"our angel Wally Rosenquist" p75
Now look at the etymology of the name Rosenquist, a Norse double-nominative = rose + twig, which brings to mind both vines and propagation by graft. Think about this in conjunction with the recent discussion of Neighbors and Inhumi, haploid/diploid/quadriploid chromosomes, and trees & vines in the Short Sun. Some remarks in the first chapter about the advanced biology of Woldercan led me to speculate that Wally may be a biological creation, a grafted propagation of some sort. His name is a suggestive hint in line with this speculation. To draw the parallel closer, angels and Inhumi both fly.
In AEG, we're led to believe that Bill/Wally are one and the same, but there are a few indications they may be two distinct entities, for example, Cassie saw Bill Reis/Wally Rosenquist in the audience during the final performance of The Red Spot, but when questioned later, whoever it was that answered denied being present. Maybe Wally is a propagation of Bill.