Character Development: He receives a lot in Part IIIafter his dad dies, and he talks to Jade's son, and realizes that he's been a bad son all along. He also meets a woman that's compatible with him, and marries her by the end of the film, indicating he's no longer a Man Child. Or at least not as much anymore.
Comically Missing the Point: He has a tendency to do this, oftentimes not realizing the seriousness of the situations they're put in, such as when Phil uses his phone to track his phone and van to see where Chow went. When Stu tries to say something about it, Alan makes it a point that losing a phone is Serious Business, because he has over 60 apps on his phone, and that it would take a lot of time to put them all back in if he lost it.
Phil: GOD DAMN IT! Alan: GOSH DARN IT! Phil: SHIT! Alan: SHOOT!
"Oh my word!"
Happily Married: In the third film, he finally meets a woman as quirky and batshit insane as he is.
Horrible Judge of Character: He outright says "The drug dealer who gave me roofies seemed like a real straight shooter!" in the first film, and insists on remaining friends with Chow. In the third film it takes being dragged into Chow's escapades, getting Doug taken hostage, and his outright murdering three people in front of him for Alan to finally realize Chow's not a nice guy.
Karma Houdini: He's responsible for almost all of the events of the trilogy, like the being drugged (twice) and most of the events of Hangover III. He gets little to no comeuppance for any of it. Of course, by the end of the third film, he finally turns his life around, having been convinced by what had just transpired over the past few days.
Made of Iron: Throughout the trilogy, Alan gets knocked out by a bare-knuckled Mike Tyson, hit in the forehead with a stun gun (and keeps going), and has his head slammed repeatedly in a car door. None of these injuries stop him for very long.
Man Child: While Alan's meek around adults he doesn't have any problem picking fights with children, which we hope is the reason he's not allowed near schools. He's also a Jonas Brothers fan.
Stu: Don't let the beard fool you, he's a child.
He did try to pick a fight with an old man for looking at his dad's car.
Part II has a Flashback sequence from Alan's perspective - It turns out that he sees himself, Doug, Phil, Stu and Chow (and only those 5!) as children.
The Millstone: He at least indirectly causes nearly every single bad thing that happens to the Wolfpack.
Morality Pet: To Chow. Of course, his so-called friendship with Chow is the only reason the Wolfpack get in trouble in the second and especially the third movie, but hadn't Chow liked him, he would probably have refused to cooperate with them and may even have killed them.
Noodle Incident: What he did that caused him to be banned from Chuck E. Cheeses.
Depraved Bisexual: In the first movie he acts Camp Gay, and in the third movie Chow hits on Stu and Phil, but in the second movie Chow says he's had a wife for fifteen years. In the third film it's also mentioned that he contacted an escort service to have both men and women sent to his hotel room.
It's All About Me: He will help the Wolfpack only if he benefits in it some way. Otherwise he either works against them, or actively tries to sabotage their efforts.
Karma Houdini: In Part III, Chow literally gets away with stealing $42 million and three counts of murder, and worms his way back into the Wolfpack's life after Alan told Chow he didn't want to be friends anymore.
Pet the Dog: Given a rare one in the third film where he doesn't kill the Wolfpack because Alan opened the trunk in the limo and gave him his gun, allowing him a "fighting chance" to survive. He even gives Alan a bar of gold, but the latter refuses it.
Faux Affably Evil: He's a drug dealer, but other than being irritated about being called 'Black' Doug he actually seems a reasonably decent guy in the first movie. In the third he could clearly care less when it looks like the Wolf Pack are going to be killed on Marshall's orders. The fact that he was Marshall's chief of security also implies he was involved in much more sinister business than his original appearance suggested.