George Russell was a real-life literary giant, who was dominant in the Irish renaissance around the turn of the century. He was committed to mystical experience, and was considered to be something of a prophet, poet, philosopher, artist, journalist, and economic theorist all at once.
In "Scylla and Charybdis," we get a taste of his literary view when he tells Stephen, "Art has to reveal to us ideas, formless spiritual essences" (9.18). In the debate over Aristotle versus Plato, it is clear that Russell is going to side with Plato, who is often regarded as the father of Christian mysticism (or neo-Platonism).
Stephen feels rebuffed when Russell does not invite him to George Moore's event, and when he does not offer to include him in his collection of young poets. Russell hears Stephen out on Shakespeare briefly, but it's quite clear that he doesn't pay the young aspiring artist much mind. The same might be said of Russell's relation to Joyce, and thus it is perhaps no surprise that at other points in the novel, Russell comes across as something of a kook, wandering the streets of Dublin talking about a twoheaded octopus.