This means it will take a lot longer to level them up than you. Many companions start at higher levels than you, especially if you're just starting your Bannerlord campaign. It's limited to civilian clothes and weapons until unlocking skill perks that allow concealed weapons. It's only limited by skill requirements for gear. The Viking helmet toggles their Battle Outfit, and the hat with a feather toggles their Civilian Outfit. However, don't forget, you can switch between their Battle and Civilian Outfit right below their name. The inventory menu on the right will still be your normal inventory for equipping companions with new armor and weapons. So long as you have a companion currently in your party, their equipped gear will appear when you cycle to them. This opens the inventory menu where you can click either arrow next to your name at the top, center of the screen, to cycle characters. This is similar to changing your own gear when you go to your inventory by pressing 'I' by default.

The first step to fully customizing your companion is upgrading their armor and weapons.