Alderpaw helped to raise Twigkit and often took her to visit her sister. He aided in several crises, such as helping gather lungwort for ShadowClan'syellowcough epidemic, and earned his full medicine cat name, Alderheart. He served his Clan well with compassion and empathy. After SkyClan returned, Alderheart strongly believed that they should have a place among the other Clans by the lake.
"I'm sure that's why they want me to be a medicine cat: not because I'm special and important, but because I'm no good as a warrior apprentice. Well, I'll just show them! I'm going to be the best medicine-cat apprentice I can be. I'll try really hard, and make Bramblestar and Squirrelflight proud. [...] I'm not really a medicine cat. I'm not... special enough."
—Alderpaw's thoughts after Bramblestar tells him he's going to be a medicine cat The Apprentice's Quest, page 62
Alderkit is born alongside Sparkkit, Dandelionkit, and Juniperkit to Bramblestar and Squirrelflight, the ThunderClan leader and deputy respectively. He and Sparkkit are the only surviving kits of their litter, and Alderkit pressures himself to be the best son he can be. He is nervous, anxious, and determined. He is given the name Alderpaw and is apprenticed to Molewhisker. Unlike his sister, who shows a natural talent for everything, Alderpaw struggles through his warrior training despite his best efforts. His father encourages him to become a medicine cat apprentice due to Leafpool and Jayfeather receiving a vision. At the Moonpool, he is chosen by Firestar to go on a quest to find SkyClan, the missing Clan. He, Sparkpaw, Molewhisker, Cherryfall, Sandstorm, and Needlepaw of ShadowClan go on the journey. At the gorge where SkyClan lived, the group finds Darktail and his group instead, and they returns back home unsuccessful.
On the way back, he and Needlepaw find two kits abandoned in a tunnel underneath a Thunderpath. Deciding they cannot leave the kits to die, they name the kits Twigkit and Violetkit and take them back to ThunderClan. At a Gathering, ShadowClan and ThunderClan fight over the kits, and Alderpaw mourns when ShadowClan separates the two sisters. He acts as a fatherly figure to Twigkit in ThunderClan and communicates with Needlepaw for the two sisters to meet in secret. He becomes friends with Puddlepaw, the new ShadowClan medicine cat, and retains his friendship with Needlepaw even if the two have differing opinions about Clan life. When yellowcough critically weakens ShadowClan and WindClan refuses to share its herbs, Alderpaw finally finds his voice and speaks against the immorality against ShadowClan. Jayfeather is impressed by his apprentice and names him a full medicine cat as Alderheart.
When Darktail and the Kin take over ShadowClan, Alderheart anxiously worries for Needletail and Violetpaw and mourns for Needletail, one of his closest friends, when he hears she died. Twigpaw brings SkyClan back to the lake, and Alderheart is finally relieved the five Clans are reunited. However, the peace doesn't last long, and he grows frustrated with Sparkpelt when she and several other cats strongly oppose SkyClan's place at the lake. Alderheart desperately misses Twigpaw when she chooses to live with her family in SkyClan, but warmly welcomes her when she returns. He develops feelings for a kittypet named Velvet when she seeks shelter in ThunderClan from a fire. However, he assures Jayfeather he is loyal to ThunderClan despite his feelings for Velvet.
Alderheart saves Puddleshine's life when he is caught in barbed wire by using deathberries. When Tigerstar discovers Alderheart's unusual methods, Tigerstar makes Alderheart become ShadowClan's temporary medicine cat as Puddleshine recovers. In ShadowClan, Alderheart becomes close to Shadowkit, one of Tigerstar's kits, and notes the kit's future as a medicine cat. Alderheart continues to advocate for SkyClan's place at the lake and is relieved when all five Clans are reunited at last.
Like the other medicine cats, Alderheart is worried when StarClan cannot be contacted, and is also worried about the Moonpool being frozen over, regularly discussing the matter with the others. When Bramblestar collapses while hunting, he and Jayfeather tend to him, and Alderheart reveals StarClan is still silent. Bramblestar's condition worsens, and Shadowpaw informs them of a cure for him. Both medicine cats try it, but it initially doesn't work and Bramblestar dies. Alderheart grieves for his father, but when he revives, Alderheart is overjoyed.
Kate said that she liked Alderheart better than Sparkpelt, because she prefers a worrier.[blog 2]
Kate has said that Alderheart does have feelings for Needletail.[blog 3]
He has kittypet blood through Firestar, and SkyClan blood through his grandfather Tigerstar.
From this day forward, this apprentice will be known as Alderpaw. Molewhisker, you will be his mentor. You are loyal, determined, and brave, and I know that you will do your best to pass on these qualities to your apprentice.
Alderpaw, is it your wish to share the deepest knowledge of StarClan as a ThunderClan medicine cat?
Warriors of StarClan, I present to you this apprentice. He has chosen the path of a medicine cat. Grant him your wisdom and insight so that he may understand your ways and heal his Clan in accordance with your will.
I, Jayfeather, medicine cat of ThunderClan, call upon my warrior ancestors to look down on this apprentice. He has trained hard to understand the ways of a medicine cat, and with your help he will serve his Clan for many moons. Alderpaw has always had a natural connection with StarClan. You chose him, and you chose well. He is loyal, determined, and smart. He has compassion and strength, a rare combination. He will serve his Clan well. Do you, Alderpaw, promise to uphold the ways of a medicine cat, to stand apart from rivalry, between Clan and Clan, and to protect all cats equally, even at the cost of your life?
Then by the powers of StarClan I give your true name as a medicine cat. Alderpaw, from this moment you shall be known as Alderheart. StarClan honors your devotion and kindness, and we welcome you as a full medicine cat of ThunderClan. Well done.