Wesley's most notable personality trait is his ruthlessness; he is "one to make the hard decisions even if he has to make them alone." This is evidenced even in his early episodes on Buffy, where he would rather allow Willow to die than to give the Mayor back an object which would allow him to complete his Ascension. He is immediately dismissed by the Scoobies as lacking in compassion, though his logic is sound (cf. "Choices"). Later examples include taking baby Connor away from Angel and apparently shooting his own father.

Wesley matures significantly over the course of both Buffy and Angel; in his early appearances he was cowardly and incompetent. Denisof described some of the guideposts he used as an actor portraying his character, "I looked to Giles a little, and considered what would be the most annoying thing for him. I thought that an irritating version of Giles would be annoying for him and also for Buffy. Wesley's purpose was to come there and point the finger and get things shipshape. He's a by-the-book school teacher. Considering what kind of person it would be who would have dedicated his life to this peculiar task of being a Watcher, and what would be the unique characteristics of somebody who had made those decisions, and then was taken out of that environment and put into Sunnydale. To Wesley that was a completely new and bizarre place." During this time, Denisof used his own backstory involving Wesley's relationship with his father as a reference for how to play Wesley, and this background story helped explain "why he was so repressed."

When Wesley is fired from the Watcher's Council, Denisof says this experience gave the character "a little shake". When he arrived in Sunnydale, he was straight out of Watcher grad school; he lacked practical experience. He was living in the ideal of the perfect way to execute his duties. I think that losing his job and going out alone roughened him up a little, lopped off some of his sharper corners. It made him more approachable and more personable, less sure of himself all the time."

By the end of Angel Season Three, following his betrayal of Angel Investigations and subsequent expulsion from the group, Wesley is deeply changed from his foppish early appearance, having disdained his glasses and shirts in favor of leather jackets and sweaters, as well as no longer shaving regularly, giving him a constant stubble around his chin. Also, his English accent is gradually softened. Denisof says, "[The modified accent] just sits on him better. As an actor, it just felt that organically the way he was changing, and it also seemed to be accurate when you consider the amount of time he's spent in L.A. that the accent could have softened. And since he isn't surrounded by upper-crust academics as he was as a young Watcher in the Academy in England, it's understandable that he is changing the way he speaks and changing his voice, his delivery, as a result of his environment."

Wesley undergoes yet another drastic personality change in Season Five when he suffers a terrible tragedy. According to Denisof, "Once Fred is killed, he really becomes unbalanced, in the way that anybody would losing what they perceive as their life partner. By the time we get to the last few episodes, he's got a handle on the grief and is functioning in a more level-headed way. But underlying it is a huge hole in his heart and it makes it possible for the decision that they make in the final episode. For him emotionally, the stage is set for a life or death battle, possibly for the last time, because at this point, there's nothing more for him to lose."

Denisof talked with Whedon about what storylines would have been in place for the sixth season (had there been one); Wesley and Illyria would have featured in an arc, and the transformation of Illyria to Fred would have been extended over many episodes and taken to a "much deeper, darker place" than it briefly was in the late fifth season episode, "The Girl in Question". Denisof continues, "They would have progressed the relationship between Wesley and Illyria in such a way it would conflict with his own feelings for Fred, in a much more profound way. And then we would have gone into the switching of Fred and Illyria and having these two people that he was having these strong feelings about. That was going to be a fairly long journey in the following season, all of which got abbreviated tremendously when [the WB] decided to cancel the show. There were things that were always in place to happen in the finale, but I don't think the death of Wesley was one of them. With the show being cancelled it did bring up opportunities for the writers. [Joss] said it would mean the death of Wesley." Whedon gave Denisof the option of keeping the character alive, but Denisof believed killing the character was right for the story: "It was very upsetting to read. It's too good a story because it hurts."