Longtail, warrior of ThunderClan, later Longstar leader of ShadowClan.
In the Boon AU, Longtail never fully believed the story about Tigerclaw's attempted murder, and thought his old friend and idol had been treated unfairly. He listened to Darkstripe's rants about it, and distrusted Redtail and Tinyfrost. When Darkstripe wanted to leave ThunderClan after Redstar became leader, Longtail agreed.
They tracked down Tigerclaw, who was living as a kittypet with Sasha. For a time the four were happy there, forming a found family in Sasha's garden. Sasha and Longtail became good friends.
Longtail went with the group to ShadowClan when Sasha needed a medicine cat to help with her kitting. Nightstar took pity on the group, and allowed them to stay. Longtail was grateful for this.
When Tigerclaw started gunning to take over ShadowClan, however, Longtail couldn't support it. He realized that Tigerclaw was everything he had denied - an ambitious murderer willing to do anything for power. When Tigerstar declared himself leader of ShadowClan and had Nightstar imprisoned, Longtail alone stood up to him, begging him to not do this. Tigerclaw proceeded to tear Longtail's ear off and have him imprisoned too.
He and Nightstar were not released until TigerClan was defeated, Tigerstar driven out of the forest, and Darkstripe was dead. Nightstar resumed his leadership of ShadowClan, and named Longtail his deputy, much to Longtail's surprise. He did this as he was the only warrior who stood up to Tigerclaw, even though it cost him greatly.
Tigerstar's betrayal and Darkstripe's death hit Sasha and Longtail both very hard, and they comforted each other through their complicated grieving. Longtail stepped up as a pseudo-father-figure for Hawkkit, Mothkit and Tadpolekit. Over time he fell in love with Sasha, but she didn't feel comfortable in ShadowClan after everything that happened with Tigerstar. When her kits were apprenticed, she left ShadowClan to roam, and Longtail loved her from afar while supporting her need for a new start.
Later in life he would succeed Nightstar and become Longstar, and lead ShadowClan to the lake territories. Hawksight, Mothfeather and Tadpolefang would go on the journey to Sun-Drown-Place - on their return journey they found their mother. They convinced Sasha to come back with them, and she ended up rejoining ShadowClan and reconnecting with Longstar. In their twilight years they fell in love properly.