Decking has always been an area we struggle with, but occasionally have a player jump in and give it a go. Some ideas in no particular order:
*On the run from a powerful AI/free sprite
*The team only knows the player in the matrix, they never see them except in AR/VR - reason why could be up to the player.
*Decker is infected with CFD and has to deal with one or more additional personalities
*The decker thinks the real world is fake and that only AR/VR are real
*Runs with a local gang of deckers
*Missing time or past - gets occasional flashes of things that may/or may not have happened
*Robin hood complex - robs from the megas gives to the poor
*Virtual friends are going missing with no explanation
*Works at a local virtual bar under another name
*Still works for Corp/KE/LoneStar/etc.
*Owes a criminal org big!
*Was a past Otaku or (one of The Whites - see Renraku Arcology Shutdown) - this would take some GM hand waving to explain
*The decker hires the team to extract them from Corp/Research facility/suspended animation/etc.
*More GM hand waving - your rigger builds anthro drone bodies for the AI decker
*Decker is actually a middle school aged kid - is a hacking prodigy, but not so great with the real world experience
*GM approval - has a one of a kind deck that has pros and cons you'd have to figure out, someone really, REALLY wants it back. Extra amusing for you if it is headware or an internal deck.

*A free sprite or AI has taken interest in the decker - good, bad? Who knows! Though I like the idea of a curious AI/sprite
*Something is stalking the decker in the matrix - or their friends. Always, just out of view.
*Journalist in disguise; publishes the teams accomplishments to their AR channel. What happens when it becomes popular?
*Has a wacky VR persona, but also acts like that in RL
*Is secretly a/an....<fill in blank>
*Conspiracy theorist is always a good go to as well....
Have fun - and good luck!
