Excellent work indeed~ You've got so many fan girls jumping x) Though I hate to be picky, but I couldn't help noticing that Swain's eyes aren't red... <:3 was it intentional~? Much love for the style~ keep the series going~
Most likely LeBlanc. So fair in the lore, she was his date at the Noxus gala that was held for Swain when he ascended to High Command; and then so far, the lore confirms only LeBlanc knows what Swain looks like without his facemask on (because in her League Judgement, her memory is meeting him to discuss their plans and they were having tea).
Then there's the old Journal of Justice hint in one of the (fake) reader letters. Where a member of the Black Rose actually sent a hidden message to LeBlanc, questioning her association with Swain.
So yeah, lore pretty much pairs them up together for certain. All Swain did was complement Sona on her singing, just like a normal person does for singers and performers they like, just like in real-life.