Mixed power levels with PCs and NPCs

[The Sundered Sky]

So I know this sort of subject comes up sometimes in RPGs, how a GM deals with characters who aren't on the same footing, point/power-wise.

In this case it's an NPC that will be a central figure(a guide, and perhaps their only consistent ally available) to the PCs in a time/space travel campaign. The NPC(perhaps there will be one or two dependents as "companions") will be a Doctor Who-ish sort of fellow. Perhaps I'll call him "The Professor" or Dr. Howe or something.

This will be much easier than many of these "power imbalance" scenarios as I'll be controlling the NPC of course. The character will be hardier and stronger than the PCs, though likely without any skills not related to science or not innately knowing when it's time for tea and biscuits. So there's some balance there I suppose.

The PCs will likely be standard early 21st century peeps. I'm not sure what skills/abilities/backgrounds to suggest to my players as useful things without going the military route. We've all played in games(White Wolf stuff certainly) where characters were supposed to be relatively normal but all the players wanted to be a d00d with a ponytail and a leather trenchcoat(to hide the katana) or a navy seal/green beret. I don't want all the players to be "obvious" military types unless "Dr. Howe" or whatever appears in the middle of a military base before whisking off with accidental new companions. And even then he's likely to turn up with kids from Alabama or Wisconsin who may or may not have seen combat yet, rather than say...Frank Castle.

I'm sure more than one GM out there has used long term session-by-session NPCs who were in some ways stronger than the PCs. I don't think I've done that before, perhaps I'm worrying over nothing?

[Gold & Appel Inc]

This is tricky business. Even if you think you're being careful to let the PCs shine, some players will insist on characterizing the powerful NPC as a "GM PC" if they're around too much. I recommend being very mindful of niche-protection, and letting the PCs totally save the powerful NPC's bacon at least once.

[Kaldrin]

I think it depends how you play the NPCs. I've got three in my cinematic group and I've used them as sounding boards for the PCs interactions. Power levels aren't really an issue, but then again I might be in GM la-la land where that's concerned. You'd have to ask my players for input on that too.

One is a very capable fighter. One is a bit of a dandy fanatic priest. The last one is a kleptomaniac former spy, who since acquiring a ring of invisibility has rarely been seen or interacts with the group on average days. All three are near the power levels of the PCs and all three have taken lead roles in combat to save the day at times. Other times they're not so great.

One time the fighter ran at full speed into an invisible barrier, knocking himself loopy for a second or two. In the same fight the priest conjured a glowing ball that was supposed to be a flashbang-type device, but it just rolled into the room and glowed warmly.

Ediit: and my second point was going to be about comparing that to how my players once had their PCs hang around two NPCs at half the combat power level. They were just a pair of merchants that had hired the group to be mercenary guards. In the end the merchants actually survived and had long relationships with the PCs despite this difference. For one thing, they might have had a spear skill at 11, but they had merchant and diplomacy at 15+...