I personally like Fjorm better, but Nowi has advantages over her in the form of Dragon Buffs, Adaptive Damage, and a refineable DC weapon. Whereas Fjorm has Ice Mirror, and is a Legendary hero so she can buff allies just by being on their team. Performance-wise Nowi is probably the better bet.
Fjorm vs Nowi
Ive been using fjorm a lot since i started playing and from what ive seen nowi seems to do a similar job, so wich one is better unmerged
nowi or fjorm (i only have one nowi and she is +atk -spd, also i have 2 Bikes so i have steady breath fodder)
Fjorm is better unmerged with base kit. Nowi is better overall, especially with 3*/4* access for IV's and merges.
Nowi is a good choice for a +10 project and definitely worth the Steady Breath. With SB/QR/Aether she will carry you through a lot of PvE. If you merge her up, she scores well enough to be used in arena. She also has middling speed and moderate attack, so with Renewal instead of QR, she is great for softening up units for bonus unit kills.
This one is pretty close.
Fjorm has significantly better res which means she can ploy; also, she's better in the AR meta with all these mage cavaliers and healers running around. But she loses to lancebreaker, which was the most common B-skill on Reinhardt. Falchion is the most common problem for Nowi, but she tends to win painfully if you play the +spd variants.
Nowi has a lot more HP which is quite useful in AR - resists Aversa, laughs at panic manors, could dish out the panic ploy herself if she wants. She can also counterattack against B.Lyn, unlike Fjorm, but in AR I often see firesweep on Lyn so maybe it's the same...but I do see a fair amount of brave bow or prf too so it's not all player phase vs the Sacaen.
She's impervious to breakers except from L.Marth, who isn't that common in my experience (unsurprising - red legendary).
Nowi tends to have fewer damage issues. Fjorm's atk is a bit low, and lightning breath also has low mt but she hits res so most melee units suffer a lot. vs ranged units they're both pretty good because most ranged units have poor def, but Nowi does have a slight advantage when it comes to the rare exception like Saizo.