The effective number of bits added by each character is log2(number of set of possible characters). If any value between 0 and 255 could be selected (with equal probability) then each character would add 8 bits of the effective key length (log2(256)=8). However, this would make the key unprintable which is a bit of a pain when you need to send the key to someone else or drop it into a device configuration.
If only lower-case characters are chosen then each character adds log2(26)=4.7 bits to the effective key length.
The best password is constructed from the largest possible set of characters, with each possible character having an equal probability of being chosen and the key is long enough to yield the required key strength.