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Playing around more with Greninja - hydro pump is underused and powerful. It can be used like G&W's up air, Mario's Fludd, or like quick attack in neutral (risky). You can use it to reposition, or to interrupt and opponent's landing.
It can be used in any angle - to stop someone's approach diagonal from you, above you, beside you, and sometimes even below you. Using it like quick attack - to go one direction and then backward to end up where you started - it a great way to use it. When you use it on the ground to go strictly left/right, you only shoot 1 pump which is great for spacing in neutral. Really great. And it does 2% per blast, so you're getting something out of it. Also, when using it grounded, you get 2 pumps for any other angle - shooting up first lets you use another shot.
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Substitute is also underrated. It's a crummy counter for sure and can be reacted to due to how slow it is, but it's a great way to punish projectiles and move around. You can perfect shield, or you can substitute+shoot yourself toward your opponent.
You teleport to the user of the projectile if you use it close enough to them - in MID-range, basically. In this range, there's usually enough endlag with projectile/item use that you'll actually get the substitute hit in. I recommend the upward angled substitute, as it's very safe and you won't have to deal with the ccrappy endlag on the move if you miss.
Outside of mid-range, Greninja attacks the item itself and it's not as useful, so make sure you're in-range. However, if you use it on a projectile outside of mid-range, you can shoot greninja UPward by holding up (also his strongest killing substitute angle) to shoot yourself into the sky to go in for an aerial approach, if you want.
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Fun.
It's also a good way to escape predictable juggling. Using it as Diddy goes in for his 2nd uair - instead of jumping out or airdodging, use substitute. Unlike most counters, you can aim Greninja's pretty radically. If you hold up, you'll shoot diddy into the air. Not sure how dependable this is yet, but definitely a tool that can only be powerful in the hands of a read-heavy player who is in control of the fight.
Greninja does have poor CQC options, but jab is great as is from-standing-position dash attack for those slightly-out-of-range OOS punishes. His multi-hit is actually really good compared to a lot of multi-hits in the game.
Standing grab is horrible so don't use it, unless you read a spot dodge - Greninja's standing grab has the active frames of a tether grab, so utilize that aspect if you want. I recommend dash grabbing only, though.
TL:DR
Greninja's normals are slow and not that great at all (at all), but he's balanced with his specials in mind. He has mobility everywhere, and thus you have to use mobility in a defensive way. Which is awkward, because usually speedsters can rely on shieldgrabs and the like and play normal footsies. But Greninja sucks at this. His footsties overall is super lackluster, which means he has to be played like a trickster.
He is at an advantageous position very rarely in battle, but he is disruptive enough and have enough 'wut' options that you have to be VERY aware of what options you have at any moment. He's more unique of a character than I thought at first, and has to be played with dedication to wrap your head around the weird crap you have to do with him, because the normal stuff you'd do with other characters just doesn't work with him and you'll get nowhere if you try and play Greninja like other Smash characters.