Obviously Naruto himself is the most common. His friends like Sasuke and Sakura also often appear. Kakashi - sensei might show up too, perhaps to give Naruto some sage advice during his bath time. And Jiraiya could be there as well, since he was Naruto's mentor and had a close relationship with him.
The main character Naruto is always there. Then there are his comrades from Team 7, Sasuke and Sakura. Hinata could also be a common character as she has a crush on Naruto. Additionally, the Nine - Tailed Fox might make an appearance in some fanfictions, either as a voice in Naruto's head or in a more physical form while Naruto is in the bath.
Hinata is often a common character. Given her long - standing feelings for Naruto in the original series, it's easy to incorporate her into an Anbu Harem scenario. Sakura can also be a part of it. She's a strong female ninja, and her relationship with Naruto in the canon can be tweaked in the fanfiction to fit the harem concept. Ino is another possibility. She has her own charm and skills, and in a fanfiction setting, she could be attracted to Naruto within the Anbu context. Also, some original female characters created specifically for the fanfiction might be included to add more variety to the harem.
Kakashi is another character that might show up frequently. He could be Naruto's mentor in understanding the sword's powers or the mysteries behind it. Jiraiya might also be present in some fanfictions. He could have some connection to the sword from his past adventures and pass on some important information to Naruto. Additionally, villains like Orochimaru might be interested in the sword and try to steal it from Naruto, which makes them common characters in these fanfictions.
Sasuke is often a common character. Given his connection to the Uchiha clan and his own powerful ocular abilities, it's easy to imagine him being involved in a story about sharing the Rinnegan. He could be a key figure in either using the shared Rinnegan for good or being tempted by its power.
Naruto is obviously the most common character since it's his time travel. Hashirama Senju is also very common as he was the founder of Konoha and has a huge influence in the Senju clan's history. Tobirama Senju often appears too as he was an important figure in the clan and in the development of Konoha.
In these fanfictions, Naruto himself is very common. Since it's a Naruto - Overlord crossover harem fanfiction, he is often the central male character around which the harem forms. Then there are female characters like Hinata from Naruto. From the Overlord universe, Albedo is often used. She has a strong personality and her appearance makes her a popular choice to be part of the harem.
One common type of original character is a rival. This rival could be another ninja who also wants to harness the power of the Kyuubi, either for good or bad reasons. They might challenge Naruto and his brother at every turn. Another could be a healer who is specifically attuned to the Kyuubi's chakra and can help Naruto when he has trouble controlling it. There could also be an old sage - like character who has been around since the time the Kyuubi was first sealed and has secrets to share.
Tenten is also often seen in these fanfictions. Given her focus on weaponry and physical combat skills, it's not a stretch to write a story where she gains muscle. Maybe she finds a magical weapon that, when she wields it, starts to transform her body, making her muscles grow and enhancing her fighting abilities.
Hinata is also often paired with Naruto. In a fanfiction, Hinata might notice Naruto flinching and use her gentle and understanding nature to comfort him. She could be the one to figure out what is wrong and support Naruto through his difficult time, which would further develop their relationship in a sweet and tender way.
Well, Naruto himself is obviously a central character. Then there are often female characters like Sakura, Hinata, and Ino. Sakura might be written as more aggressive in her pursuit of Naruto in the harem scenario. Hinata could be the shy but very loyal one, and Ino might bring a bit of sass and competition to the group.
One of the most interesting original characters is a mysterious ninja who has the power to manipulate shadows in ways even more advanced than Shikamaru. This character often appears in fanfictions where there are new threats to the Hidden Villages. Another is a female healer with a tragic past. She has the ability to not only heal physical wounds but also emotional traumas, which adds a new dimension to the story. She usually gets involved with Naruto's journey in some way.