Why is Nowi so good

She has a great BST and a DC weapon by default.

she also has magic damage

Nowi in general is a good jack-of-all-trades unit with magic damage, a built in distant counter weapon, nice defense, threaten resistance in her base kit (helping her 34 base attack which is still fair) and the ability to take on most of the popular red sword users along with or without inherited swordbreaker due to her color advantage as well.

She requires little to no inheritance to be a great unit and is quiet a common summon in general, so if her being a 5 star is out of reach for you, she can still be fairly strong (and stronger than a 5 star with enough merging actually).

Ephraim is mega rare and I have a +2, so don't ask me how I got him to that point, and Camus is only able to reach +1 with heavy hero feather investment in general. All need a bit of a makeover to be good, reliable units, but Nowi probably requires the least amount of investment.

Ephraim takes somewhat heavy investment since he's only packed with Seal and Threaten Defense, which wouldn't be so bad IF both passives actually stacked their debuff capabilities, meaning you should only choose one if you wish to keep them.
Camus has a great C passive for a Horse Emblem team and Grani's shield is rare but not very productive to his core, so he requires a little more investment than Nowi (but DEFINITELY not as much as Ephraim).

So yea, Nowi has a lot going for her, including her BST for arena scores. If adorable also counts, it's there too!