I just don't get it, sure her ult's name is windslash but it's more energy based then wind.
The thing that makes me doubt it is that Riven, isn't an assassin, Noxus has WAAAAAY more value with her in the middle of a battle commanding people then sneaking behind enemy lines.
Honestly I swear Leblanc or one of her people with the whole mimic/deceive thing is waaay more believeable
His master died from a wind technique, in a war vs Noxians in the Noxus vs Ionia conflict. Riven is a Noxian who fought in that conflict, and had a special wind technique. Seeing as Noxians don't have their commanders stay back but lead the charge, it's not hard to see that Riven is the most likely candidate.
Plot twist: Kennen killed him... because he "is the wind" and the master of Yasuo was killed by "the wind"
http://images.wikia.com/leagueoflegends/images/3/32/Kennen.move4.ogg
And Yasuo hates ninjas
"Yasuo was charged with guarding an Ionian Elder, but, foolishly believing his blade alone could make the difference, left his post to join the fray. By the time he returned, the Elder had been slain." Doesn't really imply assassination or stealth, just that the Elder died because his protector left. Otherwise, it wouldn't be a 1v1, just a quick murder. This person probably wasn't skilled in combat, so Riven just had to use her wind technique to kill him.
We (players) are not the only ones who think so. Even Yasuo suspects her. If you listen to some interaction quotes he has with riven, all he talks about is how her fighting style looks familiar or going on about his suspicions.
Taunting an enemy Riven "Which weighs more, Riven? Your blade, or your past?" "Everyone faces a reckoning, Riven." "You can't run from yourself, Riven... I've tried." "Broken sword. Broken spirit."
When an enemy Riven uses Broken Broken Wings "Wait, that technique... huh." "Huh, three swift strikes..."
When an enemy Riven uses Wind Wind Slash "Another wind blade?" "Who taught you that?"
It might be 
Kennen lol idk because in one of his quote he says "I am the wind"
Overall the person who got killed was an elder of ionia,he was a politician,NOT Yasuos master(which is a common misconception). So it is possible that the elder had no fighting skills on his own(which is why he probably needed protection). If there were others who protected the elder,then they were most likely killed too,otherwise they should have known that yasuo ran away.
Then there is the fact that the elder was killed during the Ionia-vs-noxus-war which is the war where Riven was fighting on the side of Noxus.She and her unit went across ionia to massacre the ionian troops. Riven hated how the war was more of an extermination than a glorious battle,but she still followed her orders.
So yeah,Riven was in ionia at the time of the murder,she has a wind technique and she would have had a reason to kill him(because that elder was obviously an enemy of noxus).
assassin/murder it is the fact that noxus invaded and the elder was left alone and she just walked up and executed him with out yasuo there to protect him.
Plus on the website riven is his Rival, Meaning there is history
