[Undertaker]

So some early spoiler in Chinese, not sure if it's confirmed or not, but seem pretty legit.

Spoiler for Ch. 698:

The big thing here is that Baby 5 ended up falling for Franky and stopped her assult due to a wordplay.

Baby 5 mentioned she is a morphable/changeable(hentai) human weapon and Franky responded he loves "hentai"

And the chapter title is incidentally "Hentai"

Oh, and as some have already guessed, the heart belongs to Monet