No, Suzie is NOT a Russian spy!
I've seen a ton of posts saying Suzie (Dustin's girlfriend) is a Russian spy because she sung to delay Hopper getting the key. Whilst this sounds good in theory, this makes zero sense
The odds that Suzie would need to help Dustin were so low. First of all, Dustin and the rest of the characters weren't involved at all with the Russians' key until after Suzie met due to the radio Suzie helped him make. Also, the Duffer Brothers said they debated whether to keep the scene of Suzie and Dustin singing in or not. If it was so vital to the plot because it foreshadowed Suzie being a spy then I doubt that they'd even think about getting rid of the scene.
Secondly, this is such an oddly specific situation. The Russians would have to plant Suzie in the scenario that Dustin, a child, will intercept a SECRET Russian communication, then break into their underground bunker and find a key to the Upside Down which he somehow immediately would know. Then he would team up with Hopper who found out, which is a bit more believable due to the terminator dude they sent after him. Then Hopper would have to evade terminator guy, break in to the Russian basement and then get to the key (which he'd have to kidnap a Russian to find).
THEN, the guy helping him would get Plank's Constant (a relatively well known number) wrong meaning they'd have to find it elsewhere. And then hypothetically they wouldn't look it up in a book or something, no they'd go up to the secret radio, call Dustin's girlfriend and then she would be able to enact the Russians' plan and sing Never Ending Story (which she'd have taught Dustin earlier as part of the Russians' evil scheme).
None of this is sounds like a logical continuation in advance so it doesn't really make sense at all. I highly doubt the Russians could have predicted all of this happening because surely they'd have just killed Dustin at the start considering he's a child so they could easily overpower him. Also they didn't anticipate them getting the key to the Upside Down either because they'd have just changed the password.
So in conclusion, these are such oddly specific outcomes that the Russians in the show didn't predict and no, Suzie is NOT a Russian spy in S3. This theory sounds really cool and smart but if the Duffer brothers decided to make her a Russian spy I'd be very confused.
