You should set one of your properties as the primary key. In your case this will be the ID property:

public class TestPerson
{
    [Key]
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Hair { get; set; }
    public string Race { get; set; }
    public string Measurements { get; set; }
}
public class TestPersonContext : DbContext
{
    public DbSet<TestPerson> TestPersons { get; set; }
}

Btw why are all your properties private? It would be recommended to make all them publicly available, especially the primary key.

What is more there is something called Convention over Configuration which is used quite intensively in Entity Framework. One of the conventions is the Primary Key Convention which states: