The Ultimate Imposter's true personality is somewhat unknown because they spend most of their time mimicking someone else's behavior. However when they speak briefly without disguise and it's implied that their real nature is polite and meek. They're shown to have a quite collected and serious personality. Most notably, the Imposter appears to have a truly kind and caring personality. The Imposter is also shown to be smiling in a friendly manner and being protective and supportive to others.