In my work-in-progress, my character spends part of the story away from her family and friends. This is tricky, because I don't want the reader to forget about these early characters, especially since one or two of them have important roles later on. So I had to think of some ways to keep these characters in the back of the reader's mind, even though they're not around in the middle of the story. Some strategies for keeping off-stage characters in the story:
Introduce some memories. If the main character thinks about a time or event when they were with the out of sight character, it can do double-duty. You remind the reader of the character who isn't there, as well as adding in some emotion for the main character.
Send a message. When the off-stage character sends a message, it needs to be natural, so this doesn't work if there isn't a good reason for it. But it might also be a way to include a little mystery.
Use imagination. Sometimes, if you're really close to another person, you think about what they might think when they're not with you. I can totally see a girl imagining what her friend (or her sister or her mom) might think of a situation.
Drop a name in conversation. Not exactly subtle, but if there's a natural way to do it, characters who are in the scene could mention the out-of-sight character. It could be a way to advance the plot while reminding the reader of that other character who will eventually show up again later.
Do you have any other ideas for how to remind the reader of characters who aren't front and centre in the action?