Arelle

Buff. Whoever is allied to Arelle will have advantage on all Constitution saving throws. If the player is Level 10 or above, they will also gain advantage on all Dexterity saving throws.

Regeneration. Whoever is allied with Arelle can use this ability. When Arelle's ally is reduced to 0 Hit Points, even if they are outright killed, they can use a reaction to instead drop to 5 Hit Points and immediately take a surprise action (Note, this action instills the rules of Surprise on the enemy who felled the ally.) This will cause Arelle to lose form, reducing their water to a puddle in whichever location they were. After 2d12 hours, Arelle will reform from the natural moisture in the area around the ally, purifying this water as it takes her shape. When Arelle regenerates, any fires within 30 ft. of her will be extinguished by a sudden fall of rain that affects only the flames.

Arelle is one of the 8 Frogs. These Frogs are all legendary and have a number of abilities - although not all have been discovered - that usually depend upon whether or not their ally is allied with another compatible Frog. When a Legendary Action is taken, no Frog involved in said action can take any other Legendary Actions until it has been forced to regenerate.

Salvation. When Arelle's ally is also allied to Sylka, together they can combine their abilities to save any creature the ally sees fit to protect. When someone the ally, Arelle, or Sylka can see takes damage that would reduce them to 0 Hit Points but not outright kill them, the ally can use their reaction to trade places with this creature instead. Neither the ally or the saved creature will take any damage and the attacker's turn will end. The inciting attack seemingly never happened.

Immunity. When Arelle's ally is also allied to Fugo, together they can combine their abilities to stop any attack that comes towards them. When a visible attack is launched against the ally, or when a spell that would harm the ally is being noticeably cast against the ally, the ally can use a reaction to cause Arelle and Fugo to simultaneously create a mass of fast blue water and fierce scarlet flames surrounding the ally. This will effectively act as a shield, with the freezing cold water and broiling heat of the flames changing the temperature in any weapon enough to shatter it in its wielder's hands while simultaneously nullifying the effects of any and all spells cast on the ally. This will also sever the ties of any spells cast on the ally prior to this event.