Celestial

Celestials have special evolution requirements. Either a Light Spirit with ShadowAffinity or a Shadow Spirit with LightAffinity are eligible to become Celestials. Fortunately it is possible to meet the requirements in any game as the light and shadow summoners are available to recruit regardless of your chosen element. Celestials do not evolve further and do not have the Elite special trait.

Light/Shadow Spirits that evolve into Celestials each have nine choices, but some of them differ:

Acquiring a spirit eligible for the Celestial evolution is more technically involved than any other spirit. The ability to Adapt traits onto spirits is the easiest way to access the unit as long as the Main Character has learned LightAffinity or ShadowAffinity and a base spirit of the alternate affinity is available. Another option is to summon a spirit that happens to spawn with the Catalyst trait. This peculiar trait allows a spirit to fuse with another spirit - fusing a Light Spirit and a Shadow Spirit can allow one to inherit both affinities.

Lyra and Hildy have the special ability to summon Light Spirits and Shadow Spirits respectively. Allow them to help your effort to find the ingredients for a Celestial so that your Stamina, Morale, and mental sanity can be preserved for other endeavors.

Another peculiarity is that the Celestial's first Skill (one that is not covered by the evolution) depends on from which spirit the Celestial evolved. A Celestial that used to be a Light Spirit will have Enlighten while one that used to be a Shadow Spirit will have LifeTap. A Celestial that spawns from fusing another spirit with a pre-existing Celestial via the Catalyst trait will lack Enlighten or LifeTap. This, however, may shuffle the Celestial's Level Type and accessible Skills and lead to an amusing or mournful experience.

Combat
Celestials can have a wide variety of skills and level types, allowing them to fulfill various roles in a team, but they are casters without exception. Many of their skills can have compounding effects, so if you've gone to the trouble of acquiring a Celestial, you could get more with different skills so that they can play off of each other.

SunStrike or MoonStrike can smite foes from a distance, often across the entire battlefield, for massive single-target damage. A Celestial with both can do so every turn as the two skills have cooldown of 2 turns, allowing alternate fire - the two skills actually prepare the restrictive terrain requirement for the other. Blessing can shield all allies with the Enlightened status effect and allies currently on a shadow tile or in ShadowForm by their Magic. On top of that, HolyPower can make all allies Enlightened for 3 turns.

In the terraforming role, a Celestial can use SunShine and Darkness. The former removes all shadow tiles, converts all earth tiles to grass tiles, dispels all ShadowForm, extends the duration of Enlightenment by 1, and increases global Radiant damage by the number of shadow tiles destroyed. The latter creates (Magic + 5) shadow tiles on the battlefield, dispels all Enlightenment, and increases global Dark damage by the number of grass tiles that turned into shadow tiles.

Nearly all the skills that a Celestial can use benefit from their Magic and Magic Growth. In their hands, Magic is a stat that inflicts damage on enemies, provides impenetrable protection for allies, grants beneficial effects for them, and changes the battlefield against the enemies' favor. Their only two weaknesses are that many of their skills require 2 AP to use and that they can die when an enemy so much as thinks about them, due to their low survivability. The only skill they have that they can use to defend themselves is Blink, which only a few types can access. Regular attacks swing and push enemies back a tile but this is virtually no help as the Celestial has the worst strength ratings in the game and is a waste of AP, unless you've gotten them Telekinetic. One case where they can actually survive melee is when they are Focused and can cast SunStrike or MoonStrike - two skills' main damage is ranged, but they also inflict damage on all tiles adjacent to the Celestial as well.

A player seeking to employ Celestials should consider their interaction with terrain as well. Celestial's unique passive CelestialPower provides global Radiant/Shadow damage boost depending on the number of shadow tiles and Enlightened characters on the battlefield, but increasing one benefit by themselves often reduces the other. SunStrike and MoonStrike are limited by the requirement that the Celestial must be surrounded by all shadow tiles or no single shadow tile respectively. Alternating between SunStrike and MoonStrike makes this moot until an Illuminating character or a shadow spreader moves adjacent to them. The massive benefit Celestials bring to a mixed light/shadow team can make their use more challenging and situational.

Celestials are not Elites. Just as well, since having multiple Celestials boost their utility as they can prepare their situational requirements for each other. Most boosts to Radiant/Shadow damage they bring are global.

Their powerful skills cost a lot of Mana. While their Mana pool is truly massive enough to support their repeated use, replenishing an empty pool may be costly.

Overworld
Celestials do not have inherent benefits for overworld travel.

Breeding
While Celestials can be bred, they have no real bonus toward Fertility or Virility. Still, many of their signature skills are inheritable. SunStrike and MoonStrike in particular combo into CelestialStrike.

Moneymaking
Male/Futa Celestials provide Dark semen even if they evolved from a Light Spirit. They have no specific production material from the farm.

Trivia

 * A Celestial with three MagicFocus equipped can reach a maximum HP pool of 1.