Character, symbol, sign, mark are comparable in the specific sense of an arbitrary or conventional device that is used in writing and in printing, but is neither a word nor a phrase nor a picture.
Character suggests the distinctive form or shape of such a device. It is applicable to a letter of an alphabet, to a digit in arithmetical notation, to a note in musical notation, or to a single and simple figure or diagram which is the conventional representation of such a directive or indicative value as a comma (,), a direction to delete (#), a minute in degree (‘), or an indication of G clef in music ( £ ).
Symbol may be used interchangeably with character in this sense; more typically it is employed when the meaning or significance of the character rather than its shape is stressed; thus, for each letter in the English alphabet there are various characters (as small letter and capital or italic and boldface) for use in writing and printing but each letter, whatever the character used to express it, is a symbol for a speech sound; the character ? is the symbol used to indicate that a question has been asked.
Symbol is sometimes extended to other devices than those strictly called characters, such as abbreviations, as diagrams or schematic figures, or as more or less arbitrary arrangements of numerals, letters, or other characters.
Sign, like symbol, stresses the meaning rather than the form of the device; unlike symbol, however, it is seldom interchangeable with character, either because it may be a complicated device involving many characters or because it is less arbitrary and actually suggests through its shape or form the thing which it signifies.
There is a tendency therefore to prefer sign to symbol when the device is complicated or in its form gives a hint of what it represents, either because it is a schematic representation of the thing or because it has figurative associations with the idea represented <—>, an arrow, or sign indicating direction).
Sign, however, is used idiomatically of characters indicating a mathematical operation and of those indicating one of the twelve divisions of the zodiac.
Mark comes closer to character than symbol or sign, because it carries little, if any, suggestion of reference to an idea. It is the ordinary designation of any of various characters that are used to make clear the meaning of a passage but that add nothing to that meaning or that indicate to the eye how words should be pronounced.