@SprigofThyme
Catherine is pretty naive, not too mature but she definitely knows when to be serious. Her whole series is about her breaking one of the world's biggest rules, so she's definitely not someone who always follows rules.
Arachne is a spider-lady, as in she's a woman with spider features like six eyes and six arms. She's a protagonist, though, and is somewhat of a mentor to Catherine, but is mostly a friend.
Mottie isn't so rebellious as she is unhinged from a lot of social norms due to being very reclusive. The love for pranks actually goes to Elise.
Wolfgang is a dog. A big, cybernetically enhanced dog, but a dog nonetheless.
Shiori is indeed very book-smart.
Sun is a bit of both. He's less arrogant and more has a lot of bravado. Being a prince, he does have a lot of responsibilities but hates them.
Violet is naive, yes, but she's also 7 years old, so it's somewhat appropriate.
Galatea was the result of a science project to create a human with the traits of certain types of monster.
Alex is a pretty nice guy, but he doesn't usually show it too often with having the attitude of a fairly moody teenager.
Kyler is somewhat arrogant, but it's more bravado, like with Sun. He's fairly modest, actually, he just gets into the bravado when he's in fights.
Celica is a very nerdy and introverted girl, so the name doesn't really reflect its owner here.
Haven't read 20,000 Leagues, so you could be right. My Nero's a villain who seeks revenge against Catherine for her actions during her own series. (I chose the name because it sounds cool oops)