Everyone calls Ardsley Wooster by his full or last name save fellow English adventurer and agent Trelawney Thorpe who calls him Ardsley.
Almost everyone refers to Lucrezia Mongfish as "the Lady Lucrezia" when being formal. Justified as she dropped her maiden name "Mongfish," and calling her by her married name as "Lady Heterodyne" might cause confusion with her daughter.
Outsider: Alex Jardin is usually referred to by the Loroi as either "Captain"note an honorary rank bestowed on him by a Loroi diplomat since, due to circumstances, he's both humanity's ambassador to the Loroi and military attache for his mission or "Enzin"note a Loroi Trade Language word meaning "guardian" (due to how his name translates in the Trade Language). Late in chapter 2, he tells the systems analyst Beryl, who has been the Loroi most interested in learning about him and humanity in general and who the story has given a fair amount of Ship Tease with Jardin, that she can call him Alex. This appears to greatly please her.
PHD: Discussed in strip #1151 "First name basis" and the three following strips.
In Sandra and Woo, Larisa calls her parents, Jelena and Ivan, by their first names. It's part of her laissez-faire attitude about life in general.
In Sleepless Domain, after their first night of hunting monsters together and a talk the next day, Heartful Punch gives Undine permission to use her real name, Kokoro. This is significant because HP considers "Kokoro" an Embarrassing First Name and generally prefers to go by Heartful Punch, or HP for short. Even members of HP's training club don't call her Kokoro, describing it as the "Forbidden Name", treating it like a Berserk Button, and being surprised she's okay with Undine's use of it.