Physical appearances characterize in The Left Hand of Darkness but not in the traditional way. Usually, physical appearances characterize the people they belong to. Here, Ai's reactions to physical appearances tell us about his character rather than the people he's judging. And, you know what, maybe that's for the best.
Here's an example. Ai mentions he "was galled by [Estraven's] patronizing. He was a head shorter than I, and built more like a woman than a man, more fat than muscle […]" (15.133). This statement tells us Ai is still stuck in his Earth mindset. He expects Gethenians who look like women to act like what he expects women to act like—even though they're not women, and that's a pretty gross way of talking about women, anyway.
When Ai starts to ignore these physical characteristics later in the novel, it's a sign that he's matured as a character. Late in the novel, Ai notices King Argaven looks like "a woman who has lost her baby, like a man who has lost his son" (20.31). Ai no longer physically characterizes King Argaven's sorrow as womanly or manly, but as a sorrow shared by each. Good on you, Ai, good on you.
Actions likewise tell us about the characters, but again not in the way you'd expect. By human standards, Estraven is initially cold and remote with Ai. As Ai notes, "I don't trust Estraven, whose motives are forever obscure; I don't like him: yet I feel and respond to his authority as surely as I do the warmth of the sun." (1.18). Yet, by Gethenian standards, Estraven is open with Ai and helps him immensely.
The Commensals, on the other hand, come off as helpful and friendly. They give Ai an apartment with all the amenities and take him to lunch parties. By Gethenian standards though, they're basically just tying the noose around his neck.
The actions characterization is a bit of a warning by Le Guin. Judge actions by the standard of the culture, not by the standard of your culture. At the very least, that'll help you stay away from any Voluntary Farms.
We can tell about a character from how they approach sex and love in the story. Those who tend to value others beyond what they can get from them—i.e., love—are more willing to put aside physical sex if it means keeping their loved ones safe. Others use sex to get ahead, regardless of what it might mean for the other person.
For example, Gaum tries to seduce Estraven to get leverage against Ai. He doesn't care for Estraven or Ai; he just wants to get busy so he can…get busy?
On the other hand, Estraven is really careful not to come onto Ai while in kemmer. As Ai mentions, "for [them] to meet sexually would be for [them] to meet once more as aliens" (18.31). Cryptic, yeah, but you know the feeling: sex would just mess things up. On the Ice, where survival depends on communication and partnership, such as act would only put their survival in jeopardy. So each refrains, despite their desire.