Greninja, The Ninja Pokémon. It creates throwing stars out of compressed water. When it spins them and throws them at high speed, these stars can split metal in two. It appears and vanishes with a ninja’s grace. It toys with its enemies using swift movements, while slicing them with throwing stars of sharpest water.

Overview

Greninja would have been a mediocre Pokemon on the same level of Delphox if not for its amazing ability Protean. Protean changes Greninja's typing into whatever move it uses meaning it can get a same type attack boost off everything. This leads to some spectacularly powerful coverage moves that are virtually unwallable. Things that would normally easily take on Greninja like Latias and Ferrothorn are now in danger of being annihilated by its powerful attacks. The Protean fun doesn't stop there. You can use it defensively too. Grass Knot an Electric type and tank a Thunderbolt. Hidden Power Fire a Grass type and laugh off the retaliation. Plus it has that stylish tongue scarf! Not something to be underestimated.
Positives
- Protean is one of the best, if not the best offensive abilities in the game.
- 103 base Special Attack isn't THAT strong but it is passable when all your moves get a 1.5x damage STAB boost from Protean.
- 122 base Speed is incredible. Greninja is virtually unmatched in speed with the exception of Choice Scarfers and priority moves.
- Spikes and Toxic Spikes along with Greninja's Speed makes it a wonderful offensive Spiker.
- Currently the most metagame dominant starter in Pokemon history (with the possible exception of Swampert in 3rd gen). It is very, very good.
Negatives
- Defensive stats are miserable. Greninja will easily be 1-2HKOed by neutral hits. Don't expect to survive much.
- No way to boost your offenses besides Power Up Punch. Not a big problem due to Greninja's Protean ability but it would have been nice to see the ninja Pokemon get Nasty Plot or something.

Abilities

Torrent: When HP is below 1/3rd, Water’s power increases to 1.5 times. Do not use this ever. Compared to Protean this ability is a joke.
Hidden Ability (Available):
Protean: Before the Pokémon uses a move, it becomes a pure Pokémon of that type. In other words you become the type of any move you make including STAB attack bonuses that come with it. Ice Beams that one shot Multiscale Dragonite, Hidden Power Fires that OHKO Skarmory... This thing will surprise you in how hard it can hit.

Movesets

Kecleon v2.0

- Hydro Pump / Surf
- Dark Pulse
- Ice Beam
- U-turn / Grass Knot / Hidden Power Fire
Item Attached: Life Orb
Ability: Protean
EVs and Nature:
EVs: 252 Spd / 252 SAtk / 4 HP
Hasty Nature / Timid

Life Orb Greninja is a terrifying sweeper, only stopped by a tiny handful of things. Hydro Pump hits with decent power even if the accuracy is a bit of a letdown. Surf has a bit less power but perfect accuracy. Keep in mind the power loss is noticeable and Greninja does't really have the defenses to survive a counter strike. You also have Ice Beam and Dark Pulse if you need reliability over power. Dark Pulse hits Aegislash, Gengar, Rotom and Tentacruel relatively hard and has a useful 30% flinch chance. Ice Beam has shocking power. So much power that it can OHKO full health Multiscale Dragonite as well as Latias after Stealth Rocks. The last slot is a bit of a toss up. U-turn smashes through Starmie, Assault Vest Tyranitar, Celebi, and other Greninja while scouting for counters. Grass Knot hits Tyranitar with more reliability than Hydro Pump and with more power. It also decimates Gastrodon should it try to slow you down. Hidden Power Fire takes out Skarmory in one shot and can badly damage Ferrothorn. What makes Greninja good isn't its sheer power. It is the crazy coverage that comes from having 4 STAB moves instead of only 1-2. Greninja's speed is so ridiculous that it can just throw high powered moves around without fear of retaliation.

Shinobi

- Hydro Pump
- Ice Beam / Dark Pulse
- Spikes
- Taunt / Toxic Spikes
Item Attached: Life Orb / Focus Sash
Ability: Protean
EVs and Nature:
EVs: 252 Spd / 252 SAtk / 4 HP Timid Nature

Let me make one thing very clear. Spiker Greninja is a usable set... but compared to the offensive Greninja or other Spikers like Klefki or Skarmory it simply has trouble distinguishing itself. It does a good job of forcing out a lot of Rapid Spinners and Defogers though and it is fast so it is generally guaranteed at least a single layer of Spikes. One good thing about this set is that it is fairly adaptable. If you see the opposing team is weak to Spikes and lacks a spinner / Defogger you can easily get up a few layers early game. If they are more resistant to Spikes you can instead enjoy Greninja's fantastically destructive coverage. Spikes is even a Ground type move so you can completely block Volt Switch and Thunderbolt. Ice Beam is the primary attack as Water + Ice coverage is very good and nailing a OHKO on Dragonite + Latias with a Special move is extremely solid. Dark Pulse however smashes through Jellicent and in some situations Rotom Wash. Taunt can be used to prevent opposing Spikers from doing their job as well as screwing over Defoggers. If neither of these worry you Greninja can just run Toxic Spikes. Life Orb is the suggested item but Focus Sash works if you NEED two layers of Spikes every game.

Other Options

Physical set, Choice Items, Water Shuriken, and Extrasensory.
A set worth mentioning is one that takes advantage of Greninja's Physical Attack instead of Special. Greninja actually has a rather large Physical movepool, probably even bigger than its Special. The reason Physical Greninja fails is more due to how easily Special Greninja gets perfect coverage with so few moves. The base power of Physical Greninja's moves is also quite low. It doesn't have a single Physical move with over 85 base power besides Return and Giga Impact (and item-less Acrobatics). When Flying Gem is released you could potentially use Power Up Punch / Acrobatics / Waterfall / filler to do some unexpected damage, but overall it's better to just use the Special set.
Choice Items are being mentioned simply so you never use them. Greninja's primary quality is its ability to rapidly change type and moves, getting STAB on them and gaining new resistances in a flash. Locking yourself into one move hurts Greninja's ability to do that.
Water Shuriken is Greninja's signature move. However it is very weak. Most of the time it will hit for 30-45 damage which even with STAB is very weak.
Extrasensory is the only usable Special move not mentioned previously due to its ability to smash Tentacruel without mercy. However this is pretty much the only thing it hits.

Double & Triple Battle Options

Mat Block is really cool to prevent both Greninja and its allies from taking damage. It also gets STAB Surf and Protean STAB Blizzard. While Greninja is frail it can certainly hold its own in Doubles and Triples.

Countering Greninja

Tentacruel is an okay answer for Greninja. It takes little from any move it learns besides Extrasensory. If Greninja attacks it with Grass Knot Tenta can OHKO it back with Sludge Bomb. Tentacruel also can Rapid Spin Spikes away. Ferrothorn USUALLY beats Greninja but must be aware of the rist of Hidden Power Fire. Florges and Sylveon both can easily survive Life Orb hits and can Wish + Protect off the damage. Assault Vest Tyranitar takes a solid hit from Grass Knot but can tank a few of them and OHKO Greninja back. Blissey and Chansey take almost no damage from anything Greninja can manage while statusing it and wearing the frog down with Seismic Toss. Speaking of status a statused Greninja is a dead Greninja. Between Life Orb and Poison or Burn Greninja won't last long while Paralysis destroys Greninja's best stat, its Speed. Choice Scarf Pokemon like Garchomp or Terrakion may not be able to switch directly in on Greninja but can easily revenge kill it after something else faints. Priority users like Scizor and Talonflame can bring down Greninja with little trouble provided it Protean'd itself into a type that doesn't resist the priority hits. If you don't have something that can wall Greninja it's usually pretty easy to just take it down with priority or scarfers. Even if that fails if you have something that takes neutral hits from all its moves will probably suffice, as Greninja gets by off STAB super effective hits and doesn't really hit THAT hard on its own.