Ask yourself this question: If I delete this character from the story, how much does the main plot change?
If not much changes (the plot is the same, the characters make the same decisions, character development for other characters is the same or very similar, etc.), then this character is either not worked into the plot enough or not necessary. Main characters should impact the plot in some way, whether directly or indirectly.
If only subplots are affected, then this character is a minor char...