Footstool is good when you can get it, but I feel like there might be situations where you're too far away when the shuriken hits to get the footstool but can still get this, and on fastfallers especially, they might default to a ground footstool if you try one out of FC shuriken unless conditions are perfect. Funnily enough, on some characters like Bowser, you can seemingly get a footstool out of the Fair from this instead of a DA.
Xandercosm
If you're looking for Greninja as a potential secondary, I wouldn't be super confident in recommending him just because he
is very technical. You would have to put a ton of time into the character for him to really end up a useful secondary. Even just outside of stuff like footstool tech, Greninja is a difficult character to grasp (he has a very unorthodox combination of the overall best raw mobility but some of the worst start-up in the game). Unless you're 100% comfortable potentially having to put more work into Greninja than your main in order to keep him up to speed, I wouldn't recommend it. However, if you just want to use Greninja for one or two MUs (like Bayo), then I don't think it'd be a terrible idea to pick him up, but you're still going to have to put a good amount of work into him.
As for what kind of character Greninja is, I'd categorize him as a zoning/hit&run character. He also has an extensive combo game of course, but his general gameplan tends to involve one of those two playstyles depending on the situation. His zoning can be further broken down into two categories depending on whether he's trying to play aggressively or defensively. Regardless of either, due to poor frame data, Greninja is almost always playing a reactive game, often trying to force 50/50s. Unless you know the opponent is scared, Greninja can't really commit to much (although Nair is very safe, one has to be frame perfect with it on shield in order to not get punished).
Defensively, Greninja has shurikens to force approaches, but past a certain point, he needs to basically figure out the opponent's approach and select the correct option to keep them out since none of his options can just be spammed to cover everything (although, Fair maaaaay have shifted that balance quite a bit). Basically, if one can think of Brawl Olimar as a fortress, Greninja can be thought of as a fencer. He has all the tools to cover every angle, and he can capitalize on a successful attack very well, but he doesn't have one single option or strategy that covers everything.
Offensively, Greninja relies a lot on suggested pressure, where Greninja waits right outside of the opponent's range for them to do something punishable, occasionally backing out slightly and throwing a shuriken to bait the opponent if the opponent is playing it safe. The idea, offensively, is that the opponent needs to feel pressured by us without us throwing too many moves out (if we do, we leave ourselves open to the opponent rushing in between moves), and they'll eventually respond to that pressure in a way that we can punish.
Once Greninja does get a hit, he can often get some pretty neat combos at low to mid percents, but in some cases, there will be 50/50 situations, and while Greninja can get some nice strings off of 50/50s, you may at times find damage output lacking if you're not often able to come out on top in those situations.
Probably the most key aspect of Greninja is his mobility; you almost never want to stand still and box yourself in, and you shouldn't even be afraid to leave the ground. Being able to weave around as well as Greninja does is hugely beneficial to his spacing game, and as I mentioned before, he can fill the hit&run archetype very well when he needs to. As a character with poor damage output outside of combos and terrible start-up, our mobility often ends up a more important factor than any of our moves. If you're serious about learning Greninja, taking at least 1-3 hours to become comfortable with how he moves is probably the first thing you should do. At no point should Greninja ever end up in a place slightly different than where you intended him to be, which can happen easily if you aren't fully comfortable with his movement.
If none of that scared you away from the character, then go ahead and pick him up, but remember that, in order to keep him on par with your other seconds or main, you will have to put some serious time into him.