Mapleshade.

 Mapleshade was once a loyal apprentice of ThunderClan, however, she had made one of the worst mistakes that would break our Code—falling in love with a RiverClan cat, called Appledusk. She had secret meetings with him near the river, sneaking out at night with no one noticing, but then one night she gave birth to his kits. No cat offered her help, and the medicine cat realized that her kits and Appledusk all look alike, which was told to ThunderClan, forcing Mapleshade to leave the Clan forever because she betrayed the Warrior Code. Mapleshade had no choice but to cross the river with her kits, but the river was too much for them, and drowed her kits. Mapleshade made it alive, but her kits didn't, and Appledusk blamed her for their deaths. Appledusk betrayed Mapleshade by not wanting to be with her, and even RiverClan wouldn't let her stay for one night.

Alone now, from ThunderClan, RiverClan, and even Appledusk, the tom she loved, Mapleshade had to live as a rogue. She felt a strong sense of vengance and hatred for RiverClan and ThunderClan, for the way they've treated her. However, she couldn't get revenge while she was alive, so when she died and became a Dark Forest resident, Mapleshade watched over Crookedkit and his life, rejected by his mother Rainflower, cut off from his Clanmates, and stepped in. She promised to make all of his hopes come true in return for a promise: that he would be loyal to his Clan and put it in the first place more than anything else in the world. After some time, Mapleshade trained him to become the warrior she had been, believing that she stole everything he loved the most. She forced Crookedjaw to watch his mother's death, his brother Oakheart falling in love with Bluefur, and his mate Willowbreeze and his kits dying from a sickness. After Crookedstar's daughter, Silverstream, fell in love with Graystripe, she died giving birth to his kits. This satisfied Mapleshade, knowing that Crookedstar suffered just as much as Mapleshade did. After the Clans moved to the lake, Mapleshade followed them and plotted with the other Dark Forest cats to use discontented warriors in the living Clans in the greatest battle of all. Mapleshade wanted all of the Clans to suffer just as much as she did. But we won the Great Battle thankfully and we are still alive.