You don’t have to choose the main character. That is the obvious one people think of to write as, but it could be the worse choice. They might be well-known and the readers might feel like they know them inside and out. Most readers want to know more about minor characters.
Choose someone who would be interesting to read. Then let them write the guest post. For example, if JK Rowlings had Dobby write a guest post, he might have talked about the life of being a house elf or the joy of being freed. Maybe he might have talked about meeting Harry for the first time. He is a character everyone loved which means they’d like to know more about him.
Who would your readers love to hear from? Maybe it is the bad guy. Let him talk about his villainous acts and hint at his future plans. Maybe it is a small character who is watching the action from afar. Their insight can be interesting as they are on the outside looking in. Which character would the readers love to know more about or just are curious about something in their lives?
Pick a cryptic figure or one that is quirky. It will be fun for you to write and fun for your readers to get closer to the character. It also allows you more creative writing outlets.