We still don't know how Garen, Vi, and Sona are related to Jhin.
They were featured in his spotlight and teasers, yet the lore makes no mention of them.
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They were mainly choosen because they looked good for the teasers. That isn't to say Jhin has been planning "performances" for them...
Well he Killed Sona because she was "stealing his spotlight"
Vi was probably killed because she might have tried to stop jhin from completing a kill
Garen probably the same as Vi
And zed was killed to replace shen's father as jhin had planned to already kill him before zed did.
They did explain why they were killed if you read between the lines a bit. At the moment 
kills for this shadowy cabal within the Ionian council, this was stated in lore. a rioter has also stated that Jhin always kills in fours, so he has his main target and kills 3 more random people. This may not answer your question (as we don't know whether these champs were targets or simply people he killed on the side) but I just thought I would state this.
Riot said it was just for promo. Nothing more
In the Q&A they confirmed that these are targets he wants to go for but can't for know so those are just his ideas for their "performance".