Building a Donnel

Depends on what you're wanting him to do.

+Attack is an obviously good bane.

+Speed can help him prevent some doubles, but isn't really necessary unless you're trying to quad with a Brave Lance or something.

+Defense improves his survivability agianst physical-damage foes and helps him deal even more damage with Bonfire/Ignis.

+Res, if bolstered enough, could help him become a really good mixed tank.

For his weapon, you could go with Slaying Lance for more special triggers, Brave Lance for more hits (essentially accelerating special triggers), or Berkut's Lance for more resistance in tanking mages/dragons. I personally wouldn't run Berkut's Lance unless you gave him Distant Counter, but, like I've stated on previous posts, Distant Counter isn't NEEDED to make a unit a good unit.

His A skill could be Distant Counter (for Berkut's Lance) or Steady Breath (if you have extra fodder), or Death Blow/Fury.

B is sorta dependent on what seal you have available. Quick Riposte B skill is extremely handy as well as Vantage.

C slot is flexible and is dependent on your team composition and what they need. Threaten Def is a good skill for more damage, Threaten Atk for more reliable/better tanking capabilities, or a Tactic skill if you're running a Mixed Emblem team, or a Hone/Fortify/Spur/Drive skill.

S slot could be Quick Riposte (if available) or Quickened Pulse. Could even make it Atk Smoke so your team has an easier time with taking hits.

As far as if you should promote, that's really up to you. A 4* +10 unit is more cost efficient than a 5* +10, but it's not better as far as stats (plus you don't get the benefit of feeling super good getting a unit to max rarity and merges, lol).

If you don't mind spending your next few weeks/months spending all of your feathers on Donnel merges, then go with 5*. If you want to save your feathers, then go with 4*.

For the blessing... I think that also depends on team composition, not to mention what season it is.

I'm no Donnel expert, however, I would say that on a base standpoint for any hero, one 4*+10 is almost always better than a 5* unmerged,
Save your feather for a Lukas, he will serve you better than Donnel when it comes to 5* blues

I'm going down the +10 4 star route (he's only +5 right now though), but I only use him as a hard red counter in Arena Assault if I see 2 or more tough swords on the same enemy team, so he's more specialized than he probably should be. I went with: Brave Lance, Drawback, Ignis, Triangle Adept 3, Swordbreaker 3, and Hone Spd 3. -Spd +Atk. You could run Slaying Lance and Bonfire/Ignis though, to help out his enemy phase damage, build him similar to Lukas.

look at my friend Morph's Donnel

Well my main unit is a 5* Donnel who I’m trying to +10. He’s +Speed -Res, but -Hp +Spd is probably one of his best ones. The weird thing about Donnel is that he really doesn’t have a bad boon, and any can give him an extra niche. I won’t give you my real build, since it’s kinda expensive, but here’s a good budget set.

Berkut’s Lance (+Spd Refine) (brave Lance is ok, but I feel it holds him back a bit)
Reposition
Luna/Bonfire
Death Blow 3/Fury 3/Attack +3/Speed +3
Renewal 3/Drag Back/Quick Riposte 2 or 3
Threaten Attack 3/ Threaten Defense 3/ Any Drive Skill.
Close Defense 3 SS

This set basically ensures that he’ll survive just about any melee unit, including dragons, unless they have an emerald axe and lancebreaker. This Donnel build thrives with survival as well as the enemy phase, making him a Great Wall. Donnel is a perfect all-rounder, so if you use him, you won’t be disappointed now matter how you utilize him.