Because that's the first thing you notice about Halloran, at least when he's standing up - he's large. 6'4" and well-muscled. Take a moment to get past that, then look at his face - his smile will likely be a bit sheepish. Halloran feels awkward about standing out. Hence the latte mug.
He doesn't feel awkward now. He's sitting down, blending, hanging out with Kieran and a few of tthe Kirayth. Kieran's telling some story, and his hands are moving like an Italian grandmother's; Halloran is kicking back, denimed legs crossed guy-style, faded blue T-shirt just beginning to be snug on his defined chest (Did Capri use hot water on that load of T-shirts on purpose?). His smile is soft and genuine, bringing crinkles to the corners of his eyes. Unusual eyes, too, even for his House - so pale a brown as to be almost topaz.
His hair is short - nearly stubble-shaved in back, on the sides, and the top's so short that you can only just perceive that it's curly. You wonder what it would look like long, what it would be like to tangle your fingers in it... but no, that would drive him crazy. He keeps it short. Out of his face, out of his way.
He looks distant, all of a sudden. Wistful. And if you know him very well, you know that his mind, his heart, are a few miles north, with a girl in a tower.