@benja it started out from a short Guilty Kiss diary entry from Dengeki G's magazine (seen here https://pbs.twimg.com/media/CenUHKRW8AAHXAy.jpg) and Yohane had come up with a catchphrase for Riko “With Lily’s kiss, I’ll turn you into my eternal servant” (the original phrase was リリーのキスであなたを永遠のしもべにしてあげる) Love Live! wikia they translated it into “With my lily kiss" (at least I'm pretty sure) then it caught on with the seiyuu's and a lot of artist on twitter liked the nickname and started using it a bunch, so maybe that's why it's in the game now