This is a topic to help tackle the problem of 2D characters, or 'flat' characters. This helps people to create well-rounded characters. Note, I'm not saying that I'm the best, RPer, far from it, as I intend to use resources from multiple sites and ask multiple people on this site, in order to make a fair and useful guide for everyone.
How to make a character less 2D
One way to make characters less 2D, is when you make them to pick specific things that show they're human. If this is a student, this can be something like a class they don't do very well in, or a fear so bad it's a phobia. This helps create a hole so that your characters won't be Mary-Sue/Gary-Stu. The next step is to make sure that you don't let your characters overcome it all the time. If it's something they really struggle with, then yes they will succeed sometimes, but a lot of times they won't.