No character takes up a bit. A bit has two possible values: 0 or 1.
This isn't very useful, so they decided to put 8 bits together to create a "byte". A byte has 256 possible values, which is much bigger than the two possible values of a bit. If we encoded characters as 1 byte each, we would have only 256 different characters.
Unicode uses two bytes per character, so the number of possible characters is increased to 65,536 (216). This is a much larger amount than 256.
Lowercase letters on the calculator take up two bytes because the first 256 characters (or tokens) are being used already (as commands, symbols and capital letters). I don't know why you needed to know that though… a useless piece of information for you!