As I am sure many of you know by now in the recent Pentakill: Mortal Reminder video we get to see someone kindly kick Teemo off a ledge. Who does the kicking though may surprise you.
Here we see the boots of the kind person doing so. It is fair to assume that there are no humanoids in the immediate area large enough to be wearing those boots other than the members of the band, so lets go through them one by one eh?
To start off with, Yorick. At first glance Yorick seems like a good option, being the right size and all. However when we look closely as his boots we can see that they look quite different than the ones in the gif.
The points are completely different and Yorick's boots are mostly metal.
Speaking of metal lets talk about Mordekaiser. Not a very likely option and for good reason:
We can see him boots clearly coming up to Poppy's waist, which makes his boots far larger than the ones punting Teemo.
Onwards then, we have more to cover. Since I used the word "Onwards", one of Kayle's 5 voice lines let cover her next. She is a popular option, and the one I personally believed before starting this. I say "believed" because sadly Kayle does not seem to enjoy practicing soccer with Yordles.
This was a hard to get shot. Required about 4 attempts to get a clear shot at .25X speed. Notice the metal going up the sides of the boots however, the Teemo kicker's boots are free of any such metal ornamentation.
Starting to get down to the wire here folks, and so to continue this list of no particular order: Karthus!
Do I... do I need to say more? I mean he doesn't even have legs.
Moving on: Olaf. Unfortunately this is a double strikeout as he never even gets to participate. We can clearly see and hear him hitting those rock structures around him both before and after the kicking scene. Since Teemo was kicked off a flat ledge Olaf can't be the ~~offender~~ savior. Just in case though an above shot of his boots reveal that while the most similar, his boots have straps that stick off to the side rather than wrapping around.
That leaves: Sona? Well... I mean why not?
Her skirt is most certainly long enough to hide her legs, so we can't see what is under there. The shot where Teemo learns to fly never shows above the knees, which is a pretty reasonable height to be able to hitch up a skirt to. Sona was originally a human, or at least looked like one enough to pass for years as one from birth. This means it is safe to assume that she does in fact have legs, if anything her ability to float is the real mystery. No real reason why she couldn't kick Teemo off, and if she can make herself float she can probably do the same to her skirt leaving her hands free for playing, even if she never does anything other than wave them over her instrument the whole video.
All in all the answer to the mystery of who kicked Teemo is pretty clearly Sona.