Two people named Charlie appear in Calvin and Hobbes.
The first is Calvin's barber, featured in an early two-strip story arc where Calvin was getting his hair cut. Although he once mentioned having had other barbers, this is the only one ever seen in person.
The second, more prominent, is Rosalyn's recurring boyfriend. Although never actually seen or heard, Charlie tends to get calls from Rosalyn when she is babysitting Calvin.
Whether these two Charlies are one and the same is anyone's guess.