Ludiloco
Smash Journeyman
Yes, I feel this is basic Ness mu knowledge but thank you for posting it. Ness' double jump may be huge but you have to space it super well to snap to the ledge. All characters who struggle to snap ledge struggle against Greninja's hydro pump. Ness is a bit of an exception in that his air mobility is good enough to not really matter if he gets pumped while in the middle of his jump or PKT2 animation. However, hydro pump still wrecks him if he has to pkt and isn't particularly close to the stage. At that point as a Ness main though you probably should be expecting to lose your stock in a lot of matchups.While he obviously has a tough time offstage here, I just want to point out that most Ness players won't rely on PK Thunder unless they literally have nothing left to do. The kid has a huge double jump, a long-lasting airdodge, and a solid set of aerials (all of which can kill barring Fair), so even if you get him offstage you still have to be smart about how you keep him there.
Greninja shouldn't commit too hard to keeping Ness offstage, as it's likely you'll have an easier time punishing his landing or attempt to ledge snap than it is you'll be able to hit him again offstage. Eventually you'll knock him far enough that hydro pump will be a safe gimping tool.