Reid is shown to be reckless and irresponsible when using his powers. He often uses them in inappropriate situations that could have led to the boys being exposed. Despite this, he is capable of seriousness if he feels the situation requires it. He doesn't appear to be interested in school, as we see him not paying attention in his classes, but has read 'Dreamcatcher' By Stephen King suggesting he likes to read. He is also very flirtatious, and a ladies man. This is proven when he starts to hit on Sarah when he just meets her. But fails to impress her with what he thinks is a flirty comment: "You know Sarah's my Grandmother's name."
Reid Garwin is loud and brash, pointed and sharp. He's quick to mock - everyone, including himself - but also he's very quick to help when people need it. Yes, he's shown as reckless and careless about his powers, and about Caleb's leadership - under normal circumstances. At the same time, when trouble strikes, he closes ranks with the rest of the Sons of Ipswich without much grief for the loss of his rebelliousness. He always has an often-scathing joke, but he also often drapes himself on his 'brothers' - and most of the rest of the time, his body language is exceedingly closed, his arms, most of the time, wrapped tightly in front of his body.
At a glance, his traits seem contradictory. Is he the boy who shows up creepily when a girl (who he's hit on earlier that night) only dressed in a towel turns? Or is he the boy who sees that she's scared and accompanies her to the showers to reassure her nothing wrong can be found there, without mocking her, without perving on her underdressed state, or without presuming that doing that entitles him to any favors from her? The movie doesn't quite give a definitive answer, but much can be inferred by the unity of the boys, the way that they do tolerate - even indulge - his being a hot mess (until Chase shows up to take up that title).
Reid seems to be protecting himself. Lashing out against people, especially people of authority (mostly Caleb, but also the English teacher) suggests that the trouble is mostly found at home. He overcompensates by pushing things, focusing the attention on everyone and anyone other than himself to avoid showing his own vulnerability... and caring. Within the limited time focused on him, he's shown to be protective and caring of people who seem to need it.
He's no angel - in the beginning, his idea of defusing an upcoming fight involves making a peer of his puke, and he's the one who causes an air current to raise the skirt of a girl at the bar where they hang out, and he possibly indulges in pool hustling - but even so, he rarely actually lashes out against people who're weaker or more vulnerable. The one fight he is in? Is with Caleb, someone who is his equal - or even slightly superior - in the power that sets him apart from most of their classmates.
Basically, Reid is probably sensitive, more or less a decent person, and a mix between the clown and the rebel of the Sons of Ipswich.
(Oh, and he probably borrows clothes from Tyler. Or they buy the same stuff sometimes.)