Carbuncles

Carbuncles are summons with powerful calls that provide massive Elemental ATK and Resistance (Damage Cut); they include the six actual Carbuncles and six very similar SSR summons, as well as the carbuncle-like event summon, Kero-chanKero-chan to the Rescue! All allies gain 50% Light ATK UpLight ATK is boosted
Strength: 50%Duration: 3 turns and 50% Dark CutDark DMG is cut 50%
Duration: 3 turnsDark Damage Cut..
Hidden Effect: End cooldown for one of Sakura Kinomoto's skills.

SR Carbuncles (at 3★) provide 50% Elemental ATK and Cut to the summon's Element.

eg. Garnet Carbuncle provides 50% Fire ATK UpFire ATK is boosted
Strength: 50%Duration: 3 turns and 50% Fire CutFire DMG is cut 50%
Fire Damage Cut.

Their SSR equivalents (at 4★) provide 70% Elemental ATK to the summon's element and 60% Cut to the summon's elemental advantage.

eg. Prometheus provides 70% Fire ATK UpFire ATK is boosted
Strength: 70%Duration: 3 turns and 60% Wind CutWind DMG is cut 60%
Wind Damage Cut.

Both rarities are excellent for bursts of damage or combining with other Damage Cut (such as Phalanx) to become fully immune to damage from a certain element.

4★ SSRs are thus ideal when fighting on-element, as both the attack and cut will be beneficial. They also have much higher (more than double) base stats of SRs and provide their full effect to other players via combo calls.
3★ SRs can instead be ideal when fighting mirror element or as an alternative to uncapped SSRs. Because the Sunlight Stones required to uncap SSRs are far more scarce than the Moonlight Stones required for SR Carbuncles, most new players will want to save Sunslight Stones for more influential summons, making SRs effective long-term placeholders.

Bringing two of the same Carbuncle is often viable to increase the uptime of their buffs (up to 6/8 turns). Bringing three allows for 100% uptime, but it's generally not recommended due to the opportunity cost of missing other summons.