Ninja

Ninja(忍者) are humans trained under strict codes of stealth and assassination, adquiring above-average strength and agility, as well as having the opportunity to learn both Touki and Senjutsu in the Highschool DxD: Яe-birth continuity. They're one of the points of interest through Volume 4 to Volume 8. Annabelle Hellsing, real name Sayuri, is a high-ranked ninja.

Summary
Unlike the Three Factions or any other pantheon, Ninjas instead have their own philosophy and customs, posing themselves as a neutral force that will only take action under the command of their hirers, such as japanese higher-ups or wealthy common men, even devils and angels having the opportunity to hire them with enough money. Originally common country-folks, in the traditional japanese lore it is believed that the original ninjas were farmers who were randomly hired by samurais and feudal lords in missions of assassination and robbery. Rookie Ninjas are known as Genin(下忍; lit. "Rookie ninja"), while the highest ranked ones are known as Gokujonin(極上忍; lit. Ultimate ninja). Ninjas who surpasses all the others in both techniques and power, such as Haruka, are exclusively labelled a Kaguya(かぐや様;Kaguya-sama), since they're believe to be blessed by the original goddess in the moon.

Despite being normal humans, in the Highschool DxD: Яe-birth continuity, ninjas are skilled masters of stealth and combat, agile and strong enough to go toe-to-toe against middle-class devils, also mastering techniques such as Senjutsu and Touki out of strict training regime and will-force, and with them, create their own form of witchcraft called Ninpo(忍法); lit. "Ninja Art"). High-class ninjas can also learn traditional witchcraft and become Heroes, thus living longer and becoming stronger than regular humans. Per tradition, female Ninjas who choose the way of assassination and stealth are known as shinobi(忍び), while female ninjas who choose the way of seduction and espionage are known as Kunoichi(くノ一,).

It is said that ninjas are trained in Ninja academies in hidden Ninja villages, not far away from modern society, but magically disguised in order to avoid the common eye. So far, only the Hanzo Village, also known as hidden ninja village of the Hot Springs(湯隠れの里;Yugakure no Sato) was shown and mentioned, which is also a hot-spring resort for ninjas to rest.

Appearance
Being still humans, ninjas are known to have several scars and tattoos around their bodies due to their intense training regime and failed attempts of techniques, amputees also not being uncommon due to the dangerous nature of their job. Since they also requires stealth and disguises, ninjas can be found wearing several types of clothes, from totally black gardments that completely hides their overall appearance to social, normal clothes in order to blend into the crowd.

Abilities
Ninjas have super-human strength and speed, easily surpassing those of a middle-class ninjas, and more often are masters in hand in hand and ranged combat, being also skilled in weapon handling. However, their trump card seems to be the ability to completely hide their presence, even sages having a hard time to track them down before it's too late. For being still humans, however, ninjas are still the most fragile entities, Annabelle Hellsing and Ren showing less-than-expetacular stamina and endurance, despite the former being turned into a devil.

Ninjutsu
Ninjutsu(忍術; lit. "Ninja teaching") are ninjas techniques that combines the extra energy of senjutsu and Touki plus their own form of witchcraft in order to perform Ninpo(忍法; lit. "Ninja Arts"), several kinds of techniques that a ninja uses to further improves it's performance in battle, and it can be used to assist in wars, conflicts, single battles and even mundane activities like house duties. Some of them includes:
 * Bend into the enviroment;
 * Spit fire and other elements;
 * Summon familiars regardless of lecation;
 * Create illusions and change the forms of their enviroument;
 * Etc.

Familiars
Familiars are another requirement for one to become a ninja. Much more like the devils, Ninjas have their own forest of familiars and familiar guides in order to get themselves one, which can also learn and use Ninjutsu, thus becoming Ninja Beasts(忍者獣; Ninja-jū). Powerful ninjas can also tame and pick Magical Creatures as their familiars, as Haruka, a ninja village leader, has a Qillin as her familiar.

Known Members
So far, only one Ninja Village, the Ninja village of the Hot-springs, was mentioned and seen, commanded by the Supreme ninja Kaguya Haruka. Her daughters Annabelle Hellsing, originally Sayuri, and her sister Ren, are high-class ninjas of the same village, although the former defected the ninja village after an act of vandalism.

Hidden Ninja Village of the Hot Springs

 * Haruka(Kaguya)
 * Ren(High-class shinobi)
 * Annabelle Hellsing(Formerly Sayuri; defected High-class shinobi)
 * Tsubaki(High-class kunoichi)
 * Sakura(Middle-class kunoichi)
 * Ran(Middle-class shinobi)
 * Himawari(Low-class kunoichi)
 * Rossu(low-class shinobi)

Trivia

 * The idea is vaguely based on the concept of Ninja from the naruto and Senran Kagura Series.
 * All the ninjas from the Hidden Village of the Hot springs are named after flowers; Haruka, on the other hand, stands for "Spring Flower"(春香)
 * Strangely enough, all ninjas have their own distinctive dialect despite the fact all of them came from the same place. I.e. haruka speaks in Kansai dialect; Annabelle speaks in Kyoto dialect and Ren speaks in formal keigo.
 * For choosing the path of Shinobi instead of the path of the Kunoichi, Ren is refered as a man, despite being a young woman. Male ninjas seem to have only the path of shinobi to choose from.
 * Strangely enough, Ichijou seems to have the uncanny ability to spot a ninja naked despite their presence sensors. This might be a foreshadowing about his relation with the angel Ophelia.