Not so Above It All: While he generally attempts to behave more grounded and rationally minded than most of his friends and is clearly more responsible in the majority of situations, there are several incidents that prove he is effectively capable of the same level of thoughtlessness and ineptitude
The Smart Guy: Somewhat subverted in that he's while he's academically bright and smarter than the other three, he often reveals his limited amount of social skills and general impulsiveness in particular situations.
Socially-Awkward Hero: He could be interpreted as this as he has undertaken some misguided deeds that express his ultimate lack of understanding certain societal norms. For example vocally complaining when a group of people were able to be seated in the front of a ride he and his friends had specifically intended to only to find out said people were obliged to take this space in the ride as they were disabled (somewhat justified in that he had no knowledge of their disability having never previously seen these people, and he also showed extreme remorse for these actions), impulsively placing a wig within his underwear during a date with an older woman after a prank lead to his own pubic hair being removed causing further embarrassment for him, attempting to amiably break off an effectively one sided relationship with a girl who he was never attracted to only to create a scene during his friends birthday party (this was more than partially the girls fault, manipulatively claiming that will attempted to engage in sexual intercourse with her prior to braking off the effectively non existent relationship), becoming subject of abuse after making several pretentious and patronising but ultimately not malicious claims towards a group of garage workers he would be employed by during his work experience, claiming to have engaged in several sexual encounters in order to secure his relationship with a slightly older promiscuous girl only to face more severe humiliation when his lack of sexual experience is evidenced, making wild claims of said girls opinions of him and an extreme exaggeration of their relationship to avoid being further abused by the afore mentioned group of mechanics. Other examples not listed exist within the series.
Surrounded by Idiots: He is academically more knowledgeable than his three friends and usually more grounded uptight and sensible than them (other than possibly Simon, as the voice of relative sanity alternates between the two characters depending on the situation)while he has his own moments of tactlessness, he is generally among the most rationally minded and logical of characters in the series.
Dirty Coward: Often; He immediately tells Donovan that Will is upstairs with Charlotte, mocks Simon after some guys threaten to beat him up over something Jay did, ditches his friends when some chav pre-teen girls get their brother to chase them with a baseball bat...
Drives Like Crazy: In contrast to Simon, who drives like an old woman, Jay is extremely reckless behind the wheel of his mother's car.
Freudian Excuse: The way his Dad treats him explains a lot of Jay's behaviour.
The Nicknamer: Almost never calls Will by name in the first two series, but instead some variation of "briefcase". In the third series, when Will becomes more known for shitting himself at an exam than carrying around a briefcase, Jay starts calling him by name, though still lets slip a briefcase insult/joke once in a while.
Sir Swears-a-Lot: Probably has the filthiest mouth out of the lads, although they have their moments too.
Nice Guy: Neil can be annoying due to his stupidity, but he has almost no actual Jerkass moments compared to the other lads. He is also generally polite and is quick to comfort his friends in the "arm around the shoulder and kind words" way that the others would probably deem as too "bent" or girly.
Jerk with a Heart of Jerk: Will thinks he's a good enough guy to help Neil out once (he thinks) he got a girl pregnant. He's wrong.
He does at least stop Mr. Kennedy from trying to molest Neil several times, so at least there's that.
Not So Different: Despite hating Will with passion most of the time, he genuinely agrees with him when Will argues that a fashion show is a stupid, unethical way to collect money for charity since only good-looking people are able to take part. However, he is still enough of an asshole to force Will to take care of the charity money anyways.
Sadist Teacher / Apathetic Teacher: The former against students who have managed to personally annoy or insult him, especially Will. The latter against students in general.
Straw Loser: He often appears when the four main characters seem to be complete losers to point out that they are actually not at the bottom of the hierarchy (there is a reason they're called "inbetweeners" - they're not really losers, at least not if they pull themselves together, but they are surely not popular either), because guys like John are much worse off.
However, it's inverted in the episode "Will's Birthday" where the fact that even John is invited to a party shows how uncool the four main characters are.
The Comically Serious: Or The Cringe Comically Serious if you want. Whenever he goes into unnecessary details about his sex life, he often has a completely deadpan face as if he thinks mentioning his sex life is just like discussing politics. This tends to embarrass Simon even more.