Here's a hint: it's a single letter near the middle of the alphabet.
There are A LOT of ways to be annoying in a text message.
Via: media
Like writing "lol" a lot, or at all.
You can't possibly be laughing even half as much as you say you're laughing.
Source: google.com
Or using this emoticon.
Have you ever looked at yourself making this face at the end of a sentence in real life, before? I think you maybe ought to try.
Or forming a fictional, underground, omniscient criminal coalition united under the label "A," and harassing and bullying a group of four high school girls and their loved ones.
Get a job, A!
Source: google.com