Who should i Grail ?

Grail for love. People who grail for power usually regret later.

Like the other guy said, grail who you love, not for game reasons as for most servants unless you want to solo/do some crazy 1 turn kill shenanigans, level 80-90 and the stats at that level are fine for completing everything.

Its far better to grail a servant you like but isn't the strongest to make them more viable for use than to go the overkill route and grail monster servants who only really serve for bragging rights on your friend list.

If you don’t want to grail for love, which you should lol, it’s always better to grail 4* servants over the 5*’s. Jalter would be an exception perhaps as she gets a decent stat bump from 90-100 but it takes so much QP and grails I don’t think it’s worth it. I just used 2 grails on Gorgon and that really added a good power bump to her and added some much needed HP, but I would’ve used em on her either way. All of my grails are for waifus though. All the summer gals got one each along with Ishtar and Shuten got two

Honestly, grailing doesn't make that huge of a difference. People make it seem like it's much bigger deal than it is, when in reality an SSR will get 1k more HP/ATK at best, and you'll hardly notice any significant change. Grail servants that you like, even if it's Caster Gilles. What matters the most is that you enjoy the game, and not being the strongest.

I think a grailed Kojirou will still have wildly inferior stats even at lvl 80, so gameplay-wise it will be a waste. If you love him and the grails make him more usable in game then go for it.

Grail your favorite, no matter what rarity or placement on a tier list.