Formal:
* Correspondent: This is a formal term for someone who engages in letter writing.
* Pen pal: This term is often used for someone who writes letters to a friend, especially someone they don't know personally.
Informal:
* Letter writer: This is a simple and straightforward term.
* Regular letter writer: This emphasizes the frequency of letter writing.
* Frequent writer: This is a more general term, but it can apply to letter writing.
Other:
* Epistolary: This term refers to the art or practice of letter writing, but it can also describe someone who is skilled in writing letters.
The best term to use depends on the specific situation. If you need a more formal term, use "correspondent." If you need a more informal term, use "pen pal" or "letter writer."