No comma after 'anymore', since 'riding' shares the subject 'I'. 'Suddenly from behind someone appeared' is awkwardly structured and leaves us in a comma dilemma; it should be restructured: ' Suddenly, someone appeared from behind with a big backpack'. There is no call for the comma before 'with'.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.