Nessa shouldn’t be D tier.
She cheeses earlygame like a champ.
She absolutely hard counters all forms of permanent debuffs like poison, sunder, etc.
She serves as a way to trigger on death effects without taking wounds (Woad can literally make near infinite food with her lmao)
She heals AND deals dmg with her active, something nobody else does without some sort of evolution upgrade.
She supports debuffers like Wyrmlet & Pyris.
She works god tier with ring of power & bone ring for various reasons (RoP makes her into an eternally healing tank that also does a lot of dmg from flames; BR works well with her revival as she full heals on hatching.
Yeah her healing isn’t as much as hydrim and only works on 1 unit normally (& also relies on the enemies being alive), but she is definitely usable and has interactions with a ton of stuff.