Are you a private person? Someone that people would consider shy or quiet? If so, then your phantom friend might be a shadow person.
A shadow person is one of those introverted spirits--the type you catch from the corner of your eye. Like this spectre, you probably tend to shy away from the spotlight. Instead, you thrive when you're on the side, where you can deal privately with matters. For example, you might contribute a great deal to the group project, but when it comes presenting it in front of class, you'd rather die or be invisible.
In the end, most shadow people disappear before anyone can see them. We hope that's not the case with you (hypothetically speaking), 'cause we think you might have a lot to say...and you may be surprised at how many people want to listen.