Well, there are several characters that could be used in your game to do some fanservice or to be a key part of the story. In the past, I suggested RWBY characters because, in the end, you can use several of those characters to be in the background or to help you improve your skills or stats. But it's just a suggestion, since you can use characters from Undertales or Deltarune, it's a way to ensure that people will play it. You could also include characters created by the fandom. A good exampled would be "undertale yellow", which has characters that could perhaps be added to your game very well.