This friend is an invisible fey that likes to talk a lot. A whole lot.
It tends to speak at rapid pace and the words tend to blend together making it harder to understand.
However, it is extremely friendly unless you do not give it enough attention. Unfortunately, to it, a lot of attention means a lot of your day.
It gets excited when around other people and wants to talk to you a lot, especially when they are talking.
No one else can hear it. No one can see it.
If you ignore it (or ask it what it was saying too often while you were doing something else), it gets offended.
When offended, it likes to break things or otherwise disturb them.
Like windows, glasses, plates, salt/pepper shakers or knocking over things on desks, etc.
It also has the power to make a variety of sounds and flashy lights up to three times per day each. Which it likes to do at the most disruptive times.
These lights and sounds can be seen by other people. However, it rarely uses them in an overt way.
It can use these same powers any number of times though when only used against you.
What do you do?
Somewhere in the babbling, the fey will tell you their name.
Once you know their name, you immediately gain the knowledge that saying its name will eventually banish it.
Unfortunately, it requires you to say the name one time per day for the next 18 months without missing a day.
Edit: The fey is otherwise invincible.
Attacking it will amuse it and cause it to be more annoying.