Lionheart is a golden tabby tom with green eyes, and thick fur like a lion's mane.

Lionheart was a deputy of Thunderclan under Bluestar's leadership in the forest territories. He was born to Speckletail and Smallear as Lionkit along with his sister, Goldenkit. He became an apprentice as Lionpaw with Swiftbreeze as his mentor. During his apprenticeship, he witnessed Pinestar visiting the Twolegplace, and he later convinced Pinestar to tell the clan the truth about him leaving to become a kittypet. Because of his courage and wisdom, Pinestar said that he would have named him Lionheart. Sunstar granted the former leader's quest and named him Lionheart when he became a warrior. Lionheart fathered Frostfur's kits: Brackenfur, Cinderpelt, Thornclaw and Brightheart. He also mentored Graypaw as his first apprentice, training him alongside Firepaw when the kittypet joined ThunderClan, and the senior warrior was known for his wisdom and nobility. After Redtail died, Bluestar made Lionheart deputy; however, he died due to his wounds after Shadowclan attacked ThunderClan's camp to steal his kits. He ascended to Starclan and granted Firestar and Bramblestar one of their nine lives. He also fought in the great battle alongside his former Clanmates.