Currently, I have Billy grailed at lvl 80, max fou and I'm even working on levelling his skills to max, which are at 5/4/5 currently. As you can tell, I love him a lot and I was thinking of grailing him to lvl 90 as most of the servants I plan to whale for eventually are 5* anyway, however I only have 3 grails left and would only want to grail him further if he's going to benefit from it.
It's different for each person, but most like to grail for love rather than gameplay reasons. Besides, you'll get more grails in the future, so if there's another servant you want to grail, you can just wait.
And since as NA players, we know what's coming next, you can plan carefully which servants you wish to grail on the future and how many grails will be available until then.
The people who benefit most from grails are the ones who the attack boost will scale the most, and with Billy's crit damage buffs he absolutely fits that description. You should grail him to 90 at least if you like him
As already mentioned earlier, everybody grails someone for a different reason. Luckily, Billy is both extremely useful and lovable as fuck. His skills require a lot of material (iirc at least 20 bones for level 6), but he’s worth the investment. If you really want to see what Billy is capable of at level 90 or 100, I suggest watching Honoka Green’s YouTube videos as he has Billy grailed and maxed skilled.
As a side note, I’m conflicted about grailing my 5 stars as they are already super strong so I believe grails are meant for lower leveled servants, like Billy, Cu, David, etc. I have my boi Cu at 90 and I don’t regret it at all.
