Assassin

Boy are you one lucky guy. Absolutely invest in jack. She is the best crit star gen in the game as well as one of the best assassins. Her NP also has the useful gimmick of doing extra damage to females(we all know how many female servants are in the game. She is Tier 1 for a reason and that being she gains on average 30 stars on a chain and 50+ stars on a QQQ chain. Her NP charges ridiculously fast making her a potent boss killer and she can remove buffs from bosses on demand. Honestly you hit the *jack*pot with Jack since you wont be needing another Gold Assassin for a long time

Yes, she's a beast (also best daughter)!
She's pretty much the best Assassin whose role is actually that of an Assassin's. She has top notch star gen (easily getting 50 stars on a Quick Brave chain), similarly top notch NP gen (getting 100% gauge on a Brave chain with crits becomes common), has a good niche against females, and also has great utility (evade, dispel + Heal on a short cd).

You could throw her in with Carmilla + a friend support and they will wreck Female, non-female Riders and even Demon pillars with utter ease. I'm not sure how she'd fit with Shuten since she's more Arts + debuff oriented, but I'd suggest just going with Jack for most content (she's more than enough imo) and throwing in Shuten as pinch or if you need her debuffs.