Currently there's no way to unlock different characters but you can upgrade your semibot at the service station between missions, working on things like speed and stamina which makes exploring way easier. Equipment and movement upgrades like double jump are available for purchase, definitely worth saving up for those. Your base robot has everything needed to tackle missions, just focus on getting the right upgrades to match your playstyle. The shop system lets you enhance your capabilities over time, but you'll be working with the same adorable robot throughout - which honestly, works perfectly fine for the gameplay style.

Maybe interesting: the stamina upgrades make a huge difference when you're trying to escape from threats, so consider prioritizing those early on.