Wesley first showed up in Sunnydale after Giles had defied the Watchers Council to help save Joyce and Buffy during her 18th birthday test.

He was Giles' replacement.

When she first met her new Watcher Buffy asked if he was evil 'Because the last one was evil.' Things didn't get much better from there on.

Buffy and Faith wouldn't take orders from him and Buffy would constantly defer to Giles who she still saw as her Watcher. Wesley didn't really help himself though, being insufferable and ever so annoying, ordering the two Slayers around and generally causing a nuisence of himself. When faced with various demons, Wesley revealed his cowardly streak, either screaming like a girl or willingly sharing information to save his own life.

He did have one supporter though, Cordelia, who was rather taken by his dashing English ways and he did save her life when she was being chased by Evil Willow. It was a love affair doomed to end though after the first kiss.

Then Faith turned to the dark side and the Mayor, while Buffy quit the Watcher's Council after they refused to help save Angel's life. Now Wesley had no Slayers, and his presence in Sunnydale wasn't needed, especially not by Buffy.

He did try to make amends though, by offering to help Buffy during the Mayor's ascension. And it's true that he did try although he was knocked out with one punch in the students v. vampires fight and the last we saw of Wesley was of him being carried off into a waiting ambulance. Welsey might have gone and probably forgotten but there's no keeping a stuffy Englishman down.

Wesley returned to England and was promptly sacked from the Watcher's Council for losing not one but two Slayers. Out of a job, Wesley turned to a new career and came back to the States as a rogue demon hunter. (basically he tracks demons down and kills them and no, we don't know what a rogue demon is either)

On the trail of a demon that was mutilating other demons Wesley and Angel crossed paths again. Reluctantly joining forces the pair of them tried to track this demon down together before realising he was Angel's 'client' and discovering that he had kidnapped Cordelia. They saved Cordelia's life and Wesley, amongst friends (of a sort) again decided to stay in Los Angeles and offer his services to Angel.

Contrary to popular belief, Wesley isn't totally useless. He has had the same training as Giles, knows his mythology and is quite knowledgeable in demon linguistics, it's just his fighting that lets him down.

It didn't take long for Wesley to integrate himself with Angel and Cordy and the three of them make a good team. Wesley's loyalties were tested to the limit though when Faith came to LA and was hired by Wolfram and Hart. He had always believed that Faith could be redeemed but over time he had changed his mind and saw her more as a liability. There was no love lost between ex-Watcher and Slayer, even less when Faith spent a happy day torturing him.

Even though Wesley didn't altogether trust Angel when they both lived in Sunnydale, Angel has won Wesley's trust (but they both know that if Angel does go bad again, Wesley will be first in line with a stake). He also stood by Cordelia when she was pregnant with demon babies, facing up to the demon father in an effort to save her.

Wesley proved himself to Angel when he translated some of the scroll that Angel stole from Wolfram and Hart, revealing that Angel would meet his 'Shanshu' after he'd fulfilled his obligation to the Powers that Be.

Wesley also isn't Doyle's replacement, although a lot of people feel this. He is a character in his own right (albeit a sidekick) and you'll grow to love him. It'll happen, trust us. You'll try to fight it, but he'll prove to you that he is loyal and trustworthy (even if he did bring a stake into the office). Just give him a chance to prove himself. . .