Yasuo and Riven

realtions:

Yasuo's R, Last Breath teleports Yasuo to the targeted enemy and he strikes 3 times before landing. Coincedentially, Riven's 2nd half of her ult, Wind Slash, fires 3 projectiles.

Yasuo's q, steel tempest, can be used as a knock-up after 3 uses. Riven's q, broken wings, can also be used as a knock-up after 3 uses.

Yasuo's e, sweeping blade, is a short distance dash. Riven's e, valor, is also a short distance dash.

Both champions relate to the wind in some kind of way.

Yasuo's quotes:

"which weighs more Riven, your blade or your past?" Yasuo's quotes is stating that Riven might have been the elder's killer.

"wait... that technique... huh..." "huh... 3 swift strikes..." Yasuo is indicating here that he may have recognized a similar technique to which no one had learned at the time.

"another wind blade?" "who taught you that??" Yasuo realizes that there is another person that had mastered the wind technique he has, and is demanding to know who taught her it.

other:

If you go on the L.O.L. champion grid and go to Yasuo or Riven, they are listed as rivals for eachother, so that's another big hint.

so yeah, i believe that Riven is Yasuo's elder's killer, and these similarities may show how.

Feel free to share your ideas!!!
~TheSilentDeath