An idea that I've been playing around with recently is giving Myst a butterfly 'pet'. I say it in inverted commas as it isn't a living creature, but a little robot, built for Myst as a birthday present. She likes butterflies and this is something of a physical extension to her personality. It has practical uses as well - as a robot, it contains cameras and for all intents and purposes fills the role of a small surveillance drone. It can spy on the enemy without drawing too much attention. Well, as long as they don't look at it too closely.
While it does have a few personality traits - courtesy of its AI, I'm not intending for this to be considered an additional character for the team, but the idea of a robot butterfly pet presents itself with a few gags to use throughout the series and I feel it's interesting enough to merit inclusion.