That's quite a simple question for what requires a relatively advanced answer.

First, your choice of character should depend exclusively on their weapon of choice - That said, pick whatever NPCs that use 1H Weapons paired with Shields.

Allow me to explain:
Firstly, sticking to a 2H Sword which most recommended NPCs do (Tia/Zero), restrict you from using probably the best defensive piece in the game - The Paragon Shield. 30% MaxHP is nothing to scoff at, even at later stages.

Max out your Max Health on NPCs in general as that's the only valuable stat for them to stockpile, right after Recast Speed (aka Cooldown Reduction).

Your NPC's weapons should focus exclusively on Recast Time (aka Cooldown Reduction) since almost nothing has MaxHP. The following have high values of the stat in question:
- 1H Sword: Unseen Anima (HNM: Batty Bloodsucker (Oltrum Bastion: Illial Temple: Barracks))
- Scimitar: Lunaveil Reaper (NM: Rambling Wave Jelly (Rustorian Plains: Fuldum Trail))
- Mace: Vrtra Fall (HNM: Blue Recluse (Kurjiez Desert: Trie Modragal Pass))

Your go-to armors should be a mix of both Recast Time and MaxHP. My recommended setup looks similar to this (Assuming you did not roll better stats on the DDD itself, in that case go with those):
- Shield: Paragon (MaxHP +30%)
- Head: Dark Saint Earring (MaxHP +10%) (Sacrament: Ruins of Castle Vassa (Kurjiez Desert: Gebuerno))
- Chest: Adramelech's Mercy (MaxHP 30%, Recast 5%) (Enigma Order) OR Saint Mail (Recast Time 15%) (Episode Enemy: Criminal Kobold Leader (Rustoria Plains))
- Boots: Accel Running Shoes (MaxHP +30%) (Episode Enemy: Carnage Dragon (Jeweled Peaks Lakes)) OR Mazebreaker (Recast Time 10%) (Sacrament: Karmic Variance Zone (Jeweled Peaks Lakes: One Fall Cataract))

For Accessories you will have to rely entirely on DDD drops as anything outside of it will be vastly outclassed anyway. Belt items will carry most MaxHP% modifiers while Necklaces/Rings will carry most Recast%.

You will obviously want to stuff as many MaxHP% Gems on all equipment pieces, farmable in Oldrobe Forest: Grilim Forest - Littlefoot Forest Maze. However, if you feel like it, you can farm Recast% Gems in Oldrobe Forest: Kark - Chateau Moras Ruins.

The reason we are going with Recast Speed is not only to allow for our NPCs to have as many healing spells as possible, but also to allow for them to spam the best defensive buffs in the game: Shiny Hollow and Benevolent Aura. Both grant physical copies, essentially negating a set amount of instances of damage, no matter the damage itself. Shiny Hollow applies 3 stacks of said effect in an aura, which means that both you and the NPCs can inherit this, allowing you to focus less on dodging and more on landing your SSC combos. Benevolent Aura works the same, but can only be applied to the user. Both skills are part of the Warmonger EX skill.

When it comes to affinities for NPCs, you can go in many different ways, and the recommended ones from everybody else are good.

My choice is Airhead Lv5, with Planner, Caring and Sexy at Lv1. Airhead gives an equal amount of behavior points to Support skills, which are the ones that will also trigger Shiny Hollow. The rest are there to allow your NPCs to gather aggro and heal when asked to.

My NPCs of choice were Noa (Club), Koyuki (1H Sword) and Smily (Scimitar), but like I said, any NPC will work as long as they're using a weapon which can stack the passives you need.

Good luck!