Titus is a cabbagehead, meaning that he looks like "a small man with a cabbage tied to the top of his head" (5.9). Duh. Like the boxtrolls that Arthur meets in Willbury's shop, Titus is the first cabbagehead that Arthur sees up close, because they're somewhat reclusive:
Legend had it that they lived in the caverns deep underground. It was said that they grew strange vegetables there and worshiped cabbages. This had something to do with why they tied cabbages to their heads. Even Grandfather had not seen a cabbagehead, they were so shy. (5.25)
Okay, they're shy, we get it. So then why is Titus in Willbury's shop?
Well, according to Willbury, Titus is aboveground researching gardening techniques to bring back to his group. But he's not that eager to get back to his people. As Willbury says, acting as translator, he "'would like to stay with us and help find Arthur'" (35.32). He might have a cabbage attached to his head, but it looks like he's got as much bravery as anyone.