Next best time to roll for Jack?

Grats on your Jeanne, I suppose lol. She'll be good for difficult content and a good add to an arts party, so don't be too down.

Jack makes an appearance in the Class-Based Summoning Campaign banner expected in April, that is if you want to take a shot at another 50% chance of having her. That's if you want her as soon as possible, you better be prepared to spend until you get her.

Then there's another class summon in September...

Then in November there was a Fate/Apocrypha Anime Commemoration Campaign in the JP servers, I can't say for sure if this banner will appear as it had in the NA servers, however in this banner, you're sure to get an individual rate up for Jack(and Astolfo) during the time it's active. This will be your last Jack banner for the year.

If you're aching for a ST assassin, King Hassan can be a good replacement if you have Merlin. If you don't have Merlin don't plan on getting and investing in him and King Hassan, I'd say you can skip him and simply wait for Jack to reappear instead.

So don't fear, there's a solo rate up, just not any time soon t his year... but it is this year.

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For me, best time to roll for Jack is not on her banner. She came home twice on very non-related banners with yolo rolls. Don't remember the first, but the second copy came on the Babylonia 2 banner. I wish waver would spook me like that too.