These characters have been resurrected in various ways after death.
In Naruto there is a technique of resurrecting the dead, which is Edo Tensei (Evil Reincarnation). However, there are exceptions, i.e. a character who died but was resurrected without the help of Edo Tensei.
Gaara died when One-Tail Shukaku was removed from his body. However, Chiyo’s grandmother used the forbidden technique Kisho Tensei to revive him at the cost of her own life. The old lady of Cat village did this because she wanted to atone for her sins, when she realized that she had done many things wrong in the past.
Thanks to that, Gaara was resurrected without any tailed beasts.
2. Kakashi Hatake
During Pain’s invasion of Konoha, more than half of Konoha’s population died, including Kakashi. Kakashi died from chakra exhaustion after fighting Pain.
However, after Nagato was enlightened by Naruto, Nagato used the technique of reincarnation to revive those who were killed in Konoha, including Kakashi. This ninjutsu required the use of a lot of chakra, so Nagato died from exhaustion.
Shizune was one of the characters who died during Pain’s invasion. Shizune was also the first person (besides Jiraiya) to discover the secret of Pain’s chakra-iron rods, namely the transmission of chakra from Nagato’s main body so that all of Pain’s bodies were connected.
Unfortunately, before she could pass this information on to the other Konoha ninjas, Tsunade’s apprentice was blocked by the Pain Human Path. It reads Shizune ‘s mind upon discovering that Naruto was on Mount Myōboku, and then removed her soul, killing her.
Shizune was revived in the same way as Kakashi, thanks to Nagato’s technique.
Orochimaru can’t really be killed, unless everyone with his cursed seal dies as well, leaving him with no other body reserves.
However, Orochimaru’s main body was destroyed when he was killed by Sasuke. He was later revived and freed from Sasuke’s cursed seal. Then Orochimaru’s next body was sealed by Itachi.
Thought it was over, but Orochimaru was released from the seal again, this time from Anko. After resurrecting, he transformed into White Zetsu’s body.
Ao actually died during the Fourth Great Ninja World War. Because Juubi’s attack on the headquarters of the ninja alliance was so fatal with great destructive power, he shouldn’t have survived.
However, in an unknown way, Ao reappears in Boruto and becomes part of the Kara organization with a mechanical body.
Hagoromo was dead, actually the Sage of the Six Paths who had left behind his aging physical body. However, Hagoromo never actually died, as he later lived as a spirit and still watches over the world after his death.
Even in a certain aspect, the Sage of the Six Paths can reappear in the human world, interact with living people, and even perform certain techniques.