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.
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.
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.