Blazes Boylan is the manager of a fighter, an advertising man, and a fellow singer of Molly's. He appears sporadically in Ulysses, but occupies Bloom's thought frequently because Bloom knows Boylan is going to sleep with Molly this afternoon.
Boylan is a popular man about town – a man's man you might say. In "Hades," as Bloom's carriage passes him, all the other men in the carriage salute Boylan. Bloom simply looks down at his fingernails and thinks to himself "Worst man in Dublin" (6.89). Later, in "Lestrygonians"' Bloom and Boylan nearly have a run in on Kildare Street, and Bloom has to hurry and duck into the National Library to avoid him. The third time Bloom sees Boylan, in "Sirens," he decides to follow him into the bar at the Ormond Hotel, where Boylan is meeting with Matt Lenehan. Bloom sits a bit away, not wanting to be noticed by his rival, but we see that Lenehan fawns over Boylan and the barmaids flirt with him openly. In short, Boylan is a much more popular guy than Leopold Bloom.
There is much evidence to suggest that Boylan is a simple, crude, and superficial man. In "The Wandering Rocks," Boylan is arranging to have some wine sent to Molly, and he immediately begins flirting with her. He "looked into the cut of her blouse. A young pullet" (10.134). In "Penelope," we find that Molly wasn't completely pleased with their encounter. She thinks, "I didn't like his slapping me behind going away so familiarly in the hall though I laughed Im not a horse or an ass" (18.741). We also find that, though Boylan knows Bloom and this is his first time sleeping with Molly, he doesn't make much effort to disguise it. He leaves his torn-up betting tickets in the kitchen, leaves the furniture re-arranged in their living room, and makes no effort to disguise his imprint in the bed. He doesn't even take the time to wipe off the crumbs of the potted meat he ate while lying next to Molly.
The one thing we do learn about Boylan in great detail is that he's good in bed. Molly remembers his vigor graphically, and thinks that he must have had an orgasm four or five times. The easy impression to take away from all this is that Boylan is a pretty base creature, horny and selfish, and that he's only too happy to come visit the lonely housewife Molly Bloom without thinking for a moment about the hurt his actions are causing. And that impression's probably mostly right.
When you know a little bit about where the inspiration for Boylan came from, it's not hard to see why he comes off this way. Joyce based him on two friends that he didn't particularly trust – Cosgrave and Oliver St. John Gogarty. Cosgrave, in particular, drove Joyce half-mad with jealousy when he claimed that early on in Joyce's relationship with Nora, Cosgrave had also been "seeing" her. The character of Boylan is not exactly one Joyce would sympathize with.
But let's remember a few of the important lessons we've learned from Ulysses. First, it's important to see things from a number of different points of view before you can even begin to have a sense of them. Second, empathy is heroic. Boylan's point of view, outside of the "young pullet" thought, is noticeably absent from the novel. This might actually be a case where we can use the lessons of Joyce against him. After all, we don't really know much about what Boylan is like. Just a reminder to keep our judgment of characters in check.