My recommendation: don't have "other PC" roles at all. The default Paranoia assumption - a team of Troubleshooters go shoot trouble - is based on the intrigue and backstabbing possible in a full party. A single GM cannot replicate that, especially as you'd blur the troubleshooter treachery with your depiction of a hostile and treacherous world.
So don't try. Instead, design a mission brief around the fact that there is only one character, and use that to introduce yourself to the world. Arrange for threats of treachery and backstabbing by the player's superiors and subordinates, not his peers - NPC peers should instead establish secret society intrigue, resources or connections. Remember - treachery is something that comes from your allies; to get the tone for Paranoia right in this case you'll need to establish some allies before you start making your player suspicious of them.
Instead of the usual fetch quest, put the player in charge of interacting with people - interrogating a set of suspects in a minor offence, or handling a critical admin blockage in an important service firm.
Some basic principles of Paranoia apply even more strongly in your case:
The Computer is always watching.
With fewer PCs, Our Friend is an even-more critical NPC. Decide how you're going play it and how accessible the Computer will be, and stick with it. I recommend leaving Our Friend more offstage than usual, and having requests go through NPC superiors, with only a few instances of direct communication. The Computer is too godlike to play well in a single-player game.
Status is everything.
Establish superior officers near the character's own level. Consider making them Orange, not the default Red, so you can have NPCs and scheming above and below them to replace the scheming of their peers. That also lets them commandeer lower-level assistance. Make it clear that they live in a shark tank of circling predators waiting patiently for that hint of blood in the water.
Set up rivalries between Yellow superiors, so they can learn to pick sides for personal advantage. Blue and higher clearance citizens should be mostly offscreen - godlike entities who float in to reorder the world, then vanish.
Everything is treasonous and dangerous, but you should do it anyway.
With fewer PCs you need to establish resources and scheming at the peer level. Emphasize the usefulness of the player's secret society as a source of contacts and resources. Establish (by NPC example) how valuable a mutant power can be if used sparingly and precisely. Then establish (also by example, preferably at the player's hands) just how bad it can be if you get caught...
The bureaucracy is everywhere.
Launch quickly into the Complex red-clearance tape mountain. Let them requisition tools, get frustrated finding clear rooms, and enjoy trying to fill out a request for treason accusation form form before they can execute anyone. This is an obstacle navigable by the lone PC, yet capable of producing the right level of alienation.
A general note:
Paradoxically, Paranoia's open embrace of total GM power and its unusually hostile attitude to the players makes it harder to GM, not easier. The system will not restrain you from making mistakes, so you must restrain yourself. Paranoia done right is a transcendently beautiful experience, but it's easier to spoil through GMing overreach than any other game I know. For a one-on-one I recommend avoiding the normal open sadistic glee of the Paranoia GM - the world is flagrantly hostile, so the GM really doesn't need to be.
I don't normally recommend Paranoia as a one-on-one game - it just wasn't designed for it. It can be done, and well, but it won't be an easy GMing exercise - be sure you set your goals carefully. I'd most recommend doing this as an intro exercise to lead into group games - if you're always going to play one on one, there are better choices.