What's wrong with Stheno? Shitty stats or Noble Phantasm? (Never played the game)
For the longest time, Stheno was one of the worst Assassins in the game.
She had only two skills, one that increased her NP bar and another that charmed only male opponents (in a game where most non-Servant enemies are non-gender).
Her NP was single target and had a pretty decently high rate of instant death... but only to male enemies (again, most enemies are non-gender) and also charmed them if they were male if the IK failed, as well as a universal defense down debuff. But it also dealt zero damage.
Basically, she was a joke character who could easily build her NP bar but her NP was all but useless.
Recently though she was buffed with a third skill which massively improved her usability, which was a party attack buff that doubled in effectiveness if the party members had the Divine trait (of which there are a lot of Servants with that trait). Basically, she could buff a Divine party with 40% attack for 3/5 turns, including herself (most Servants with a double party buff can't affect themselves) which meant that now she was a pretty good buffer Servant.
Her NP still sucks though, which is why many people consider getting the first Stheno to be amazing and any more copies (which only serve in strengthen a Servant's NP) are useless spooks.