Generally speaking, the game is balanced for a group of 6, and there's little reason to have less. That being said, Solo playthroughs are possible at lower difficulties or through extreme cheese.

WotR offers two kinds of companions : companions proper, unique NPCs with their own appearance, quests, etc, which are pre-built when you get them (but can be leveled as you wish from then on), and mercenaries, fully custom NPCs.

Companions

Foreword : hardcore beware

Companions are generally optional, and can die/be resurrected without limitations (beyond the usual spellcasting requirements).

However, a few areas or encounters are specific to companion quests. Said companion must be part of your group to enter the area, and he will be treated as the main character would for the duration, meaning any death is a rip with no opportunity for resurrection.

Particularly nasty areas

Prologue : Seelah is mandatory and this early in the game is quite at risk of one-shots. Selecting a pet-based class (Cavalier, etc) soonest, especially if you don't plan on stringing Camelia or Seelah along, can go a long way to avoiding a rip.
Gwerm mansion (Camelia) the last room has multiple sneaking Babau, more than strong enough to oneshot a character with a good backstab. Keep Camelia back.
A2 cemetary (Sosiel) You get to chose between entering the church from the front (very rough fights vs archers) or sneak in from the back/rooftops. If you should the latter, Sosiel will rush in like the braindead personality he has, and your entire group will start the fight surrounded. Very bad options all around.
A3 dragon hunt (Greybor) Greybor's suicidal tendencies reached such levels of stupid that the game was officially patched. Now he's just regular level of stupidly suicidal. When you zone in to the tower after tracking down the wounded dragon, lay down Resist and Protection to fire on him (you should be able to blanket the entire group anyway) before proceeding into the area. As soon as you have control of the hobbit, carefully shuffle him way way back.
A3 "investigate the murders" (spoiler) : depending on dialogue choice, you may have to 1-1 one of your (now former) companion. This procs as you enter Drezen, so possibly need a rest.

Availability


NPC	Recruitable during...	Missable ?	Condition
Seelah	Prologue	No
Camelia	Prologue	No
Lann	Prologue	No
Wenduag	Prologue	Yes	Side with her (to the exclusion of Lann) OR Chose Lann over her, then find her much later
Woljif	Start of Act 1	Yes	Get him out of jail. Trivial. Note: will desert you before assaulting Drezen and rejoin when you travel to Greengate
Ember	Start of Act 1	Yes	Talk to her in the Marketplace.
Nenio	Start of Act 1	No	Guaranteed encounter while traveling
Daeran	Early in Act 1	Yes	Breadcrumb quest given near Desna's church
Sosiel	Start of Act 2	No
Regill	Early in Act 2	Yes	Go rescue him when prompted. Second-worst fight of A2 if unprepared.
Arueshalae	End of Act 2	Yes	Solve Desna's Riddle before assaulting Drezen, then free her. OR You'll meet her significantly later for another opportunity
Greybor	Act 3	Yes	Hire him after the dragon businiess is concluded.

In addition to the above, "standard" companions

Liches gain access to dedicated undead companions, including a half-customizable mythic companion (who doesn't count against party limit)
Azatas are granted Aivu as a mythic companion (doesn't count toward the party limit)
Anyone strong enough to endure the blandness of Sosiel may eventually receive the blander Trever, along with his atrocious template.

Details

Camelia

Seelah

The first companion you'll get, and a true sister-in-arms all game long unless you deeply embrace evil. Her CHA is sadly low, but her stats are otherwise excellent, and she gets along with everyone.

Most likely your frontline at least for a while. Keep to her martial prowesses (Paladin 11 for instance) and raise that Charisma whenever you can.

Lore: being a Paladin of Iomedae, you'd think she would go well with Angel, but really anything good or fun is completely fine by her, and none of them have much relevance.

Lann

Mixed bag. He's an excellent archer, very strong in act 1 and overpowered in act 2, then falls out a bit (as most archers do)

On the other hand he's whiny, insecure, and female characters may well end up locked in a romance by pure accident (poorly worded dialogue options, improved with recent patches).

Zen archers builds are basically archers on rails and steroid. You just can't go wrong with them as a pure class.

Lore: not much of relevant, although he's an advocate for Lawful choices.

Ember

The quintessential strong-but-not-powergaming witch. Can fit in and benefit any party, will remain relevant the entire game (with a noticeable drop during midgame).

Her story is quite interesting (YMMV) and feature multiple possible endings, from heart-warming to absolutely heart-breaking.

Lore: Just a precious little ball of sad sunshine.

Arueshalae

The strongest companion. Her points-buy equivalent is off the chart, and her kit excellent, at least assuming you get her early (level 8), which takes a bit of work. You'll get more opportunities later, but by then she'll be level 13 with more questionable feats.

She can largely overperform even an optimized main character competing around the same role, and will take care of all your thieving needs better than anyone (except maybe a dedicated Azata) could.

Lore: her story is most relevant to Azatas or (very alternatively) Demons, being currently stuck between the two worlds herself.

Woljif

Essentially a thief/mage hybrid.

Daeran

Mechanically he's, by far, the best healer in the game. This is almost detrimental, as he is "too" good at healing (curative) but not as good as you might want at preventing injuries (buffs, etc).

Lore: Well written without being outstanding, he's not nearly as shallow as he tries to appear. Loves mischief and will get along greatly with Woljf.

Sosiel

The alternative to Daeran when it comes to divine casting. Nowhere near as good a healer, but somewhat better at buffing.

Lore: Incredibly dull personality. Not relevant to anything.

Regill

The breakout character. Everyone should do at least one playthrough with Regill.

His stats are just bad though, from ability distribution to class kit.

Lore: He could almost make Aeons sound lenient. Pair with a Lawful Mythic if you want him thoroughly pleased.

Wenduag

Chaotic Evil backstabber and proud to be. Her stats are everything you could want for the role, and her romance was surprisingly interesting.

On the other hand, there will be several occasions where she'll consider betraying you and/or leaving the party, so best enjoyed with a tiny bit of metagaming.

Lore: Lich is probably the best fit, followed closely by demon. Anything radiating power/danger and she's moth to a flame.

Nenio

Perfect stats for a wizard (20 INT, Spell Focus, Spell Penetration), the best wizard school and arguably the best kit. Pretty much always pick her unless using MC/mercenaries.

Lore: Oddball character in terms of personality, unique enough that she should be picked for at least one playthrough.

Greybor

Summed up perfectly by "ego writing checks your body can't cash". Quite obviously the fruit of the 10k$ kickstarter tier : a shallow self-insert with thin attempts at depths and no charisma.

Mercenaries

Recruiting a mercenary has a cost, scaling with your own. Beyond that however, you can recruit however many you wish.

For each of them, you'll be able to create them as you would you own main character at the start of the game, with the exception that they have an 18 points-buy ability pool instead of 25. In short, they are slightly weaker than the MC.

Companions or mercenaries ?

For the most part, companions have excellent stat, often far in excess of what a mercenary could, which offset their often sub-optimal default template.

Companions make for the best experience (by far) when just trying to play through the game, hardcore-mode or not. Even Core difficulty can undoubtedly be completed with nothing but regular companions.

They should be your default choice.

Mercenaries enable things that companions just can't, such as testing synergies between mutiple classes the companions couldn't do, or testing multiple unique classes in a single go (kineticist, monk, ...)

Min-max enjoyers may wish to rely extensively on mercenaries as well, but this should not be a necessity under any normal playing conditions.