5 Star Servants' NP levels

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5 different 5* > np5 5*

Furthermore it's 1% chance to get a 5* and it might not even be the servant you are aiming for.

NP1 is generally plenty for 5*s Jalter in particular really doens't need np2 due to being primarily crit focused though. It is worth noting Jalter does have the hardest hitting NP in the game though and thus gains more damage than most from getting np2 so if you really like it its more or less worth while to go after her in any case everything past np3 is just vanity though with hugely diminishing returns.

Except if you really confident in your luck to get another copy of jalter, better save for another servant that may be able to support your jalter or compliment your jalter....

"luck" is not needed to NP5 JAlter, only cash (stays true to all gacha games tho)

Whether or not it's worth it roll for more Jalters really depends on how rich you are. A higher NP level obviously means that your NP does more damage, so it's never a bad thing. But suppose that you've gotta choose between rolling for another Jalter now, or rolling for Merlin in December, then you should 100% save your remaining quartz for Merlin. A Merlin + a NP1 Jalter is way better than a NP5 Jalter with no Merlin.

Now to answer your question about servants in general, some servants are more worth it to try for higher NP levels than others. For example, Jalter's main draw is her huge buster crits, moreso than her NP. So she's quite functional even at NP1, and increasing her NP level isn't going to make a particularly noticeable change in her gameplay value. Whereas a servant like Squirtoria would benefit much more from having higher NP levels, because her main draw is the fact that she can spam her NP frequently. So I'd try to take that type of thing into account before deciding whether or not to try for more copies of a certain SSR.