Kato in terms of narrative and looks. It’s hilarious how much she appeals to me in general, talk about pandering specifically to my tastes. But I don’t come here to publicly thirst (not that I judge such),so I’ll leave it at that.
For gameplay, Stheno. Shinshin’s still easily outclassed and unnecessary even post-SQ, Paraiso isn’t godawful but her competition is for her and Shiki still exists after all. Kato is 12 Cost for hipster HoH and nothing else (it’s honestly kind of sad IMO how slowly a L100 Kato takes to kill merely Coatl in this solo video), when David is 5 fewer Cost for the same and a Charisma and deceptive bulkiness, on top of NP1 Robot-ninja-doll-mom-wife being literally directly comparable to Phantom not-in-niche and unremarkable within it (never mind anywhere near optimal where ‘difficult’ enemies go). Certainly Nito can at least loop, but that requires more expensive tools which narrows her broader applicability, even before factoring in that you would be hoping at all for her not to IK some trash mobs at the start.
Now, Stheno isn’t anything inherently top-tier for sure, but she’s a support and supports are fundamentally valuable. Being a Divine-specific buffer (+40% ATK is nifty), who can also charm and lower gauge and further raise effective ATK (+60-70%) assuming you’re not using her as Poster Girl fodder, is definitely broadly applicable in a number of reasonable situations - buying a turn where your attacker isn’t harmed and someone gets 3T of 40+% ATK buffs is literally always nice, gauge reduction has its obvious upsides, and that’s before getting into the slew of powerhouses and/or fan favorites that happen to be Divine: AGoldie, Ishtar, Paru, Eresh, Bryn, Medusa, Iskandar, Golden, Melty, Shuten, Mama, and several others. You could do worse.