Best friends are a gift to be treasured for as long as you can keep them. Your character will probably develop some very interesting relationships over the course of their life, but it is the best friend who will have the largest impact on your character outside of a family member or love interest.
A best friend can serve as a partner-in-arms, a trusted confidant, a confident supporter, a well-timed comedian, and even a loving self-sacrificer if the need should arise.
On the flip side, they can also lead or join your character in some pretty devious capers which might put your character in harm's way.
If your character has, or had, a best friend, don't forget to give them a name as well. Next, it would be a very good idea to mention why this person is your character's best friend. Explore some of the fun or otherwise significant memories that they share with each other.
If they are no longer best friends, explain what happened between them that caused the ending of the relationship. Was there a betrayal of trust? Was the friend killed by a rogue red dragon? Did they turn to a life of crime and are now serving a life sentence in the dungeon of the local castle?
There are many things that can make or break a friendship. And some of these things may have a significant impact on your character and how they feel or react to certain situations or events. If your character has a new best friend within your current adventuring party, how are they different from or similar to your past best friend(s)?