So champs that have an A as the fourth letter in an ability are
Blitzcrank, Caitlyn, Cho'Gath, Diana, Elise, Evelynn, Ezreal, Fiddlesticks, Hecarim, Jarvan IV, katarina, Kog'Maw, leona, Lux, Maokai, Nami, nocturne, Orianna, Pantheon, Poppy, Rengar, Rumble, Ryze, Singed, Sion, Skarner, Sona, Vayne, Vel'Koz, Vi, Xerath, Ziggs, Zyra
Of those, nine are marked by Riot as assassins:
Evelynn
Katarina
Nocturne
Pantheon
Poppy
Rengar
Vayne
Vi
Xerath
Of those, four have a method of hiding
Evelynn
Nocturne
Rengar
Vayne
As for the J and the 6, no idea. Vi is strongly associated with both, her name is the roman numeral six, she was abandoned at six years old, and her enemy is Jinx. But she just received a new skin, and doesn't meet any other criteria. Evelynn and Vayne both have cooldowns which are six seconds regardless of cooldown, (6 seconds to use condemn on vayne, six second passive on Evelynn). Vayne does not even have a single J appear anywhere on her champion page. And none have a J appear in a name or anything of particular importance in their lore or kit.