Rarity

Allies come in different rarities. Most allies in the game will be SSR, with a few exceptions.

N Rarity allies are all from the common pool, and there is one of each character.
Allies of N rarity have low stats and no intimacy rooms, meaning there is no intimacy bonus. They are mostly used for their Exploration Passives.
There are 15 N Allies.

R Rarity allies, like N rarity ones, are from the common pool and have one of each character.
These allies have identical skills as the N rarity allies, but their stats are higher and they have intimacy rooms. Finishing the rooms will give a total of 50% stats on top of the clear rewards, but there will be no H-scene. Potential can only be raised to 6, but the full 100% bonus can be obtained, making these allies easy to use early on. They also have Exploration Passives to aid Explorations.
There are 11 R Allies.

SR Rarity allies of each character are also in the common pool, with the same skills as the R and N allies, but with higher stats. Some events have SR characters as well.
SR allies require a Crystal Core to be raised to 5☆. Their intimacy rooms give 50% bonus stats, and the final room will be an H-scene. Their potential can also be raised to 12 rather than 6 for a 100% stat bonus. SR allies typically are easier to raise to their maximum stats, so they will often be suggested over the higher rarity allies.
There are 18 SR Allies.

SSR Rarity allies of each character also exist in the common pool, but most are obtained through events. These are usually extremely specialized allies with a wide array of skills.
SSR allies require two Crystal Core to be raised to 5☆. Their intimacy rooms give 70% bonus stats, and both the second and the final room will be an H-scene. They also can have their potential raised to 12 for a 100% stat bonus.
There are 117 SSR Allies.