Guardianstar is a short tom that has a black pelt with a white, front paw. He has sleek, short fur with ice-blue eyes.
When Guardianstar was a kitten, he lived in a twoleg nest with his mother and siblings and was named Tiny. His mother, Quince, let him and his siblings, Ruby and Socks, outside and he wanted to prove himself to his siblings, so he went beyond the fence. Outside of the safety of his housefolk's fence he ran into a ThunderClan apprentice, defending their territory, and the two young cats fought and then the apprentice stopped to explain that his name is Ravenpaw and that he was training to be a warrior of ThunderClan. Ravenpaw smells the scent of two ThunderClan warriors and tells Tiny to hide, but it is too late. Snowstar and Redtail emerge from nearby bushes and question Ravenpaw about Tiny. Snowstar compliments Tiny on how well he fought Ravenpaw and what a good hunter he was. Redtail says that Tiny is simply a kittypet and he should be taken back to his Twolegs.
Snowstar offers a place in ThunderClan for Tiny, and was given until sunhigh to make his decision. He tells Ruby and Socks that he met three wild cats in the forest and invite them to join in ThunderClan. Ruby and Socks deny the offer and claims that he is not telling the truth. Tiny leaves his twoleg nest at sunhigh and meet Snowstar with Ravenpaw at highrock. Snowstar announces that she has allowed Tiny into ThunderClan, many of her Clanmates are infuriated. A young warrior named Runningwind taunts him, reminding his Clanmates that Twolegs would invade their territory because of Tiny's collar. Tiny lunges at Runningwind and gives him a torn ear. Runningwind tears off Tiny's purple collar with his teeth, and Snowstar accepts the torn collar as a sign that Tiny should be a Clan cat, and grants him his apprentice name, Guardianpaw, in honor of Guarding his dedication to being a ThunderClan member.
Guardianpaw is mentored by Brackenclaw and Redtail, until he eventually gets a very shocking choice for an official mentor: Snowstar. The Clan is very surprised, as it is a high honor, but Brackenclaw remarks that Guardianpaw wasn't being punished by feeding the enemy warrior, Yellowfang. Snowstar chose him because of his exceptional skill that exceeds the other apprentices. Brackenclaw continually bullies him, as do the two older and more experienced apprentices, Maplepaw and Applepaw. Ravenpaw stays by Guardianpaw's side all the time and they become best friends. Graypaw also becomes their friend, overcoming some of his timidness, but Brackenclaw, his taunting mentor, is still a big target of his nervousness.
Guardianpaw encounters a ShadowClan cat named Yellowfang, who is now living as a rogue since being unfairly driven out by Raggedstar, ShadowClan's leader. They battle, and Guardianpaw wins, injuring Yellowfang's leg in the process. Instead of turning her over to Snowstar, Guardianpaw feeds her before feeding his own Clan. After they both have eaten, they are caught by a ThunderClan patrol, which takes Yellowfang into custody. Guardianpaw's punishment for breaking the Warrior Code is to care for the grumpy former medicine cat.