How do I counter a suicide Reaper rush as Protoss in StarCraft 2?

It sounds like your problem is less, "I need a counter" and more "How could I have known?"

First off, you said you scouted him but he proxy'd his Barracks (hid it outside of his base). Let's go over the ways you could have still seen this coming:

• Usually not seeing a Barracks is a good sign he's Proxying.
• Watching for a very early Vespean Refinery is usually a sign of some sort of early Marauder/Reaper play
• Finally, many top Protoss players default into 1 Gate -> Cybernetics Core to set themselves up to stop this.

What else could you have done?

Some guide will suggest skipping that first Zealot, but if you can squeeze him out (Zealot has a 33 sec build time compared to the Cybernetics Core's 60) he serves as a good distraction to keep the reapers off your probes.

Make sure to save some extra chrono boosts to boost out a Stalker. While early game you should be using these on Probes, once you start building the Cybernetics Core stop using them and save them for the Stalker.

Another problem you had was a Bunker slipping in. There are a couple ways to stop this, but most prominently walling off your ramp. You can do this with your Gateway/Cybernetics Core, or leave space and place that first Zealot there. Alternatively, you can build an extra pylon closing off that small space with plans to kill it later.

Once he does start building the Bunker, you can use either your Stalker or even your Probes to try and kill it during production (or even better the SCV building it). The follow up will usually be a large number of Marauders so you can either push out Zealots to deal with this (its unlikely he'll have concussive shell this early), or a Sentry. With the Sentry you'll want to use force field at the bottom of your ramp to keep him from moving up and gaining sight. You can then use your Stalker to prevent Reapers from giving sight and to target the helpless Marauders.

In any case you should consider your next move. Are you going to go Robo? get out an early Immortal. Or do you want to throw up 2-3 Gate and out produce him (as he's seriously cut production).

One of the things you'll notice is that as you continue to play, the time it takes you to get out that first stalker will rapidly decrease. A Reaper rush really trades on the opponent not seeing it, and not being very experienced.

In general, whenever you play against Terran as Protoss, get a Stalker as fast as you possibly can because there's a good chance he's going to reaper rush you. Stalkers eat Reapers for breakfast and will solve the whole issue.

You really need to work on your speed and your macro to make sure you get everything timed perfectly to counter the Reaper. Build a Pylon at 9, a Gateway at 10-11, a Cybernetics Core as soon as the Gateway finishes, and a Stalker as soon as the Cybernetics Core finishes, using Chronoboost on the stalker if necessary. Usually the assimilator fits in somewhere after the Gateway.

From there it's easy to transition into pretty much any midgame you want.

1st thing: scout his base. Often it is a good idea to do that early. Especially on 4 player maps. If you scouted opponent VERY early - just look what is going on - and leave base for half minute. Check it later... don't lose your scout!

Signs of reaper rush: early refinery + 2 barracks both with tech-lab. 2 barracks with 1tech lab means - plans for marauders+marines. 1 Barrack + 1 Factory - tanks/hellions. if you see only 1 rax and not factory - it means a proxy rax somewhere.

In case of ANY suspicious: don't skip 1st zealot. You will have at least some defense from reaper/bunker harass.

And next thing - stalker. Actually, I don't see a lot of reasons to skip zealot: you will save 100 minerals... and have potential issue to deal with 3 quick marines. If they kill 2 probes - then you "save" failed.

I would add one more thing: if you scouted early reaper rush - you can celebrate. The reason for that: you know - than you are prepared. Reaper rush is very consuming thing: it consumes a lot of minerals/gas/time (3 reapers is minimum amount for harass, that means 150 min, 150 gas; produce 150 gas in early game means time that you could produce 150 minerals + 75 minerals for refinery + almost 22 minerals that SCV can produce instead of building refinery): totally almost 400 minerals). If you detected and denied reaper rush it means that you are 400 minerals ahead. Use that either for FE or for harass.