A back-up friend - a friend who is barely a friend but gets called in when your

a friend who is barely a friend but gets called in when your main squad is either busy, gone, or too lazy to show up

You text them: 'Hey, I need someone to go with me!' They reply: 'Sure!' Then you realize you’re the only one at the movie.

They're the one who shows up to your sleepover and eats all the pizza, then leaves before the fun starts.

They're the one you call when your main friend is out of town, but then they take 30 minutes to reply.