Is there a term for a person with an obsession on a single character or letter? For example: A person that is obsessed with the letter "X".
.A. If the obsession embraces all the letters in turn, then they are Abecedarians. People who like alphabets.
.B. Someone who puts a chosen letter at the beginning of a cluster of words is practising Alliteration. Someone who alliterates is an 'alliterator.' Not usually monogamously loyal to one letter. There is a famous alliterative poem by AAWatts, which starts with…
AN Austrian army, awfully arrayed,
Boldly by battery besieged Belgrade.
Cossack commanders cannonading come,
Dealing destruction's devastating doom. to...XYZ&A
Only a couple of single-letter infatuations have a name:
.C. iotacism Merriam-Webster Medical:
a speech defect marked by use of the sound \ē\ in place of other vowel sounds.
In language studies iotacism refers to the vowel shift e.g. in modern Greek where ei, oi, i, and ee all sound like ee, or i.
.D. lisp (Oxf Dictionaries)
in which s is pronounced like th in thick and z is pronounced like th in this:
The bigger editions say this is sometimes an affectation, simply a stylish preference for th, v, ff; for sibilants. Also some choirs are taught to avoid final,whistling esses with a lisp.
.E. Chiastic ought to mean someone who likes Chi or X but it is only used for crossover patterns. Lambdaism shows a preference for arrow shapes or Lambdas Λ, but not usually LL sounds or letters.
Could they be an Xophile? Why do you need such a word anyway?
The term for the person is 'monomaniac'. The term for the type of obsession is 'monomania':
An obsession or abnormally extreme enthusiasm for a single idea or subject ....
[monomania. (n.d.) Farlex Partner Medical Dictionary. (2012). Retrieved December 8 2015 from http://medical-dictionary.thefreedictionary.com/monomania .]
No more specific term than 'monomaniac' exists for somebody obsessed with single letters, nor for somebody obsessed with the individual single letters. If you have the desire (need?), such terms could, potentially, be formed.
Unfortunately, for the case of an obsession with single letters (without regard for which individual single letters), the term 'graphomania' (and, to a lesser extent, 'graphophilia') has a pre-existing more general meaning:
an obsession with writing.
[graphomania. (n.d.) -Ologies & -Isms. (2008). Retrieved December 8 2015 from http://www.thefreedictionary.com/graphomania .]
'Glyph' might offer a suitable combining form, as in 'glyphomaniac' or 'glyphophiliac':
3. A symbol, such as a stylized figure or arrow on a public sign, that imparts information nonverbally.
[glyph. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved December 8 2015 from http://www.thefreedictionary.com/glyph .]
To form words expressing the idea of 'an obsession or abnormally extreme enthusiasm' for individual letters, you'd have to look to the names of the letters, then combine them with '-maniac' or '-philiac' to designate somebody obsessed with the particular letter.