"I can't make all dead cats appear. Needletail told me about StarClan. I've never seen a StarClan cat. I think I can only sense cats who are still tied to the living. They've stuck close to us. They can't move on until they've done what they feel they need to do. Like Needletail. She will only find her way to StarClan when she has finished what she started here."
Ghost cats cannot be seen by the average cat, and can only be seen by those descended from the Sisters, or by other ghosts themselves. They appear as transparent versions of their living selves. However, it appears it is actually possible to scent a ghost, evidenced by Fidgetflake who had detected ThunderClan scent from the ghost of Squirrelflight, though he could not place who it belonged to.
On the contrary, the Sisters believe that the ghosts guide them similar to the Clan's belief of their StarClan ancestors. Members of the Sisters go to the residence in their death and guide the living. The Sisters can also communicate with the ghost cats, whether they are related to them or not, as was the case with Blade.
In some cases, ghosts are "attached" to a descendant of the Sisters and can only be seen by them, such as Tree with the former ShadowClan cats, or Rootpaw with Bramblestar. Though, Spiresight states that not every cat with Sister blood can see every ghost. While only the living cat can see them without effort, it is possible to bring them into the living world for all to see, as Tree did with his ghosts at the lake and Rootpaw bringing Bramblestar into the ShadowClan camp. However, doing this puts a strain on the cat and cannot do it for long.
Tree is able to make the spirits of Mistcloud, Needletail, Dawnpelt, Beenose, Birchbark, and Lioneye appear to the Clans, and the spirits urge Rowanstar to have faith in his leadership. However, Rowanstar refuses and steps down from his leadership, reverting back to his warrior name and annexing ShadowClan into SkyClan.
Pebbleshine appears as a ghost to her kits, Violetshine and Twigbranch, as well as her mate Hawkwing. After finally being able to meet her kits, and informing Violetshine of her unborn kits, Pebbleshine is able to move on to StarClan, having reunited with her family and finding solace.
Rootpaw continually sees a dark spirit around Bramblestar, though he is unable to deduce what it is. Tree also tells Wrenkit and Bellaleaf of their deceased kin around them, causing Rootpaw to get angry with his father. Rootpaw later sees an apparition of Bramblestar in SkyClan territory. The spirit calls for help, but Rootpaw runs away in fear.
Bramblestar's spirit struggles to acknowledge his corpse is possessed by an impostor, who begins a tyrannical rule of ThunderClan. Rootpaw initially ignores the leader's pleas for help but eventually obliges after realizing how much of a threat to the Clans the impostor is. He informs his father Tree, who decides that in order to not start a potential war, that Rootpaw needs to keep is the ability to see the ghost secret.
Eventually, a secret meeting is held between the Clans to discuss the change in Bramblestar's personality. Tree lies to them that he is able to see Bramblestar's ghost to protect Rootpaw, and they come to discover that the Bramblestar leading ThunderClan is an impostor. Rootpaw later informs Bristlefrost and Squirrelflight of Bramblestar's situation, though both are skeptical, both she-cats come to realize that Rootpaw is telling the truth.
In an effort to bring Tigerstar to believe Squirrelflight's claims of Bramblestar being a ghost, Rootpaw, under the guise of Tree doing it, makes the spirit of Bramblestar appear in the ShadowClan camp. The entirety of ShadowClan realizes that Bramblestar's body is possessed, and Tigerstar decides to rally against the impostor.
After almost being killed by Bramblestar's impostor, Shadowsight meets an old friend, Spiresight, who is now a ghost. Caught between life and death, Shadowsight becomes a ghost as well. Rootpaw later makes the ghost of Bramblestar appear for the Medicine cats of the Clans to prove that the Bramblestar leading ThunderClan was a fake.
After Rootpaw fails to make the ghost of Bramblestar appear for a while, Tree tries to use a ceremony to bring back the fallen leader, but this fails. Tree concludes that the longer Bramblestar is out of his body, he will slowly fade away.
Stemleaf, Dappletuft, and Conefoot become ghosts in the aftermath of a failed attempt on the impostor's life, where they are killed and, unable to reach StarClan, are stranded in the ghost residence.
Shadowsight, utilizing his ghost abilities and the help of Spiresight, is able to make his presence known to the alive cats, where Rootspring and Dewspring retrieve his barely-alive body. Later on while recovering in SkyClan's medicine den, Shadowsight, as a ghost, goes onto ThunderClan territory and into their camp. There, he witnesses the impostor abuse his warriors before revealing he is able to see the ghost of Shadowsight. The impostor leaves the body of Bramblestar, revealing his spirit, though Shadowsight can only make out his dark blue eyes. The impostor reveals that he has dealt with the spirit of Bramblestar, and spirits can make their way between afterlives if they're clever enough. However, Shadowsight is forced to return to his body before he can get any more information out of the impostor. Shadowsight later eats a deathberry to journey to the Dark Forest. Now again a spirit, he frees the trapped spirit of Bramblestar, who had since disappeared mysteriously.
The cats who perished in battle for ShadowClan become ghosts, as they too cannot reach StarClan. Rootspring spots their ghosts staring forlornly at their living Clanmates shortly after the battle.
On their way back to SkyClan camp from the vigil, Rootspring and Tree encounter a ghost. The ghost reveals herself as a kittypet named Cheddar. The she-cat is surprised on how both of them can see her, as most cats cannot. Rootspring asks her if she had seen other spirit-cats, and she explains that she sometimes sees ghosts of cats she doesn't know. Cheddar describes them as cats that look scared, and that they fade in and out, like they can't decide where they want to stay. The cats part ways.
Later, Rootspring and Tree relay the information to Leafstar. Leafstar decides that this should be said in front of all the Clan leaders. Rootspring explains this, and this leads to the leaders deciding that they should go to the Sisters for help.
When the Sisters are brought back, they perform a ceremony to summon the ghosts roaming the forest. To Rootspring's horror, he witnesses them writhing in pain and lashing out uncontrollably. He catches sight of Ashfur at the edge of the SkyClan camp and not suffering at all. Ashfur later denies making the ghosts suffer.
Ashfur later reveals to Shadowsight that he can control other spirits to make them enter and leave Bramblestar's body, demonstrating with Spiresight and Bramblestar's spirit.
Moonlight reveals to Squirrelflight that ghost cats guide the Sisters, similar to what StarClan does with the Clan cats. She also reveals that all members and those descended from the Sisters are able to see ghost cats.
After Squirrelflight nearly dies, she briefly becomes a ghost while her body heals. She meets Blade, a former kittypet who is now a ghost as well, and Squirrelflight becomes paranoid that she will remain stuck in the unnamed residence. She goes to find Tree, who discovers Squirrelflight and helps her guide her way back to StarClan so she can go back to the living world. Snow and Tempest later reveal that Moonlight has become a ghost after her death, now guiding the Sisters like their ancestors.
Throughout his time with the Sisters, Earth is unable to see ghosts at all, though he longs for Stream's spirit to come to him after his death. The Sisters are able to commune with Haze and Stream following the storm, while Earth strains to see Stream. Moonlight discusses this with him, comforting him that it sometimes takes time. Later, Earth is trying to convince Moonlight to let him stay with the Sisters for a bit more, citing that he cannot see ghosts. Moonlight assures Earth that Cloud, an older leader of the Sisters, let a tom stay longer than 6 moons, and that leaf-bare was the hardest in memory that the Sisters have faced.
After Root's death, his spirit warns Tree and Pebbleshine to flee from a coming fox. Later, Tree sees Root again and apologizes for his death. Root tells him not to blame himself, expressing pride in what his son has become.