SPOILER WARNING GUYS IF YOU DONT WANNA KNOW WHO THE CHILD IS YET!!!!!!!!
soooooo it's basically guts and casca's son that was absorbed by Griffith... meaning it's basically Griffith but he is called the moonlight boy and this is the only time Griffith i vulnerable because he becomes this little boy... if you get what I mean. So casca has this motherly affection to him and even though Griffith said before that he finally is rid of his feeling for guts (by feelings I mean caring not anything else) the moonlight boy is his vulnerable and human side that guts and casca made. And if u don't know how Griffith is guts and casca's child as I said before... it because Griffith had raped casca and his evil seed/sperm fused with guts and casca's developing child and she gave birth prematurely to that evil thing that was previously haunting guts. HOPE I GAVE SOME CLARITY!!!!!!!!