Possible Clues:

• One of those in the centre of the left and the right is an unsociable type
• What a long-distance runner needs to be?
• Reclusive star, perhaps
• Not the outgoing type
• Company avoider
• One to hold hands? On the contrary, a solitary soul
• Infrequent socializer
• Solitary type of person
• Not exactly a company person
• Introvert musician, perhaps