But neither the logic of the story nor your imagination particularly suggests any character traits.
At this point, think of the pantheon of Greek gods: Zeus, Hera, Poseidon, Demeter, and so on. Pick one. Preferably one dissimilar to your hero. And then assign this bit character a trait from that god -- ill-tempered like Ares, drunk like Dionysus, cerebral like Athena, and so on. It can be any trait that that god conjures up for you; the point is to steal it, not to make the character like that god. (You can if you want to, of course, but making the character that particular archetype is not needed for this technique.) If you have more than one colorless bit character, pick a different god before you repeat.
Any list of character types would probably work, I dare say. Indeed, I have heard a writer describing her use of one of those "The N Character Type" books, to ensure that not all her characters turn into Apollonians. (It's only one of two uses I've heard of for such books. The other is to check your characters against them, and if you can identify the type, you need to round out your character more; then, that's more for major characters.) But the Greek pantheon is the one I use.
A warning: you assign the trait to bring the character to life. Sometimes it works too well. You have a sentry snarling, "So what do you want?" because he's ill-tempered, and he suddenly says, "And I should appear here, there, and the other place as well, because I'd be doing something while the hero's doing his thing. What, you think I sit around waiting for him?" Which may, or may not, derail the story -- which may, or may not, take it off into new and useful directions. But then, any number of things can do that.