Deasy is a minor character and is represented as something of a fool. In the "Nestor" episode, he lectures Stephen about money and English history, but it is clear that Stephen does not respect him. Much of what Deasy says is cliché and unoriginal, and he makes several mistakes as he attempts to quote various people. When Stephen responds elusively, telling him that God is a shout in the street or that history is a nightmare from which he is trying to awake, Deasy does not know what to make of it.
Aside from the fact that Stephen does not seem to respect Deasy as a figure of authority, there are a few other things that make Deasy an undesirable character in Stephen's eyes. First, he is an English sympathizer, in contrast to Stephen whose sympathies are with Ireland though he resists the pull of Irish nationalism. Second, Deasy is a blatant anti-Semite. He explains that Jews are the national problem in England, and chases after Stephen to tell an offensive lame joke about why the Irish never persecuted the Jews. Though Stephen does not respond, we get a sense of his reaction through the description of Deasy's laugh as something quite disgusting.
A bit later, we get another perspective on Deasy. When Stephen delivers his article on foot and mouth disease to the newspaper, the other men joke about the article. It becomes clear that Deasy is not a very well respected man around town. One of them starts to go on about how awful Deasy's wife is to him, and Stephen, who openly resents Deasy in earlier chapters, begins to take a more sympathetic view of him. Though Stephen does not condone them, he gets a sense of where some of Deasy's stronger prejudices come from.