There are mortals who are very worthy foes to supernatural monsters like yourself, and not all of them are Slayers or sorcerers. There are also semi-magical fighters and killers of various kinds, including the legendary Ninjas of Japan. Some of these mystical assassins have offered themselves in service to Vampires of various rank, pledging their lives to their undead teachers in exchange for the opportunity to take part in the deeper stratagems of the night.
There are differing opinions on the actual goals and ideals of the Ninjas, who are few in number in the modern world, but it’s generally agreed that they are a traditional group of living people who devote their lives to committing murder for hire. (Among other things.) They spend years perfecting skills, abilities and technologies that give them extraordinary stealth and lethality. Vampires have capabilities that would serve Ninjas well, but these supernatural powers can only be approximated, and only through years of close study of the nosferatu.
It is entirely probable that Ninjas work with Vampires specifically to prevent conflict between the two groups, to gain inside knowledge about Vampires that cannot be had by any other means, and even to create the possibility of Vampirism becoming part of their Ninja group’s arsenal of weapons. Ninjas show every sign of considering the “Pet” relationship with Vampires to be a net benefit for themselves and their own plans.
Unlike a genuinely monstrous Pet, such as an Abomination—which could potentially be seen as simply a “creature on a leash,” no different from a human keeping a beloved animal as their companion—a Vampire keeping Ninjas as Pets is something that a lot of non-Vampires find very disturbing. Ninjas are, after all, people, and keeping a Ninja as a Pet is something that has a lot of troubling connotations on top of the basic moral issues it raises.
The fact that Vampires simply use “Pet” as a nickname for “favored pawns” and that they don’t even really exert a supernatural or physical influence to force most Pets to work for them—especially not Ninjas—does somewhat blunt the off-putting nature of Vampires having Pet Ninjas. The fact that Ninjas (and some other Pets) willingly come to Vampires to enter their service as a kind of apprenticeship has some ameliorating effect on the situation, as well.
But to this entirely legitimate set of complaints, Ninja-keeping Vampires ask only if keeping Ninja Pets is worse than the fact that Vampires are reanimated corpses that murder people every night by drinking their blood. Or if it makes them look any less admirable than their unfortunate habits of using fear and violence to defy reason and extend their own existence long after their natural deaths.
For what it’s worth, Vampires sometimes make an effort to find out their Ninja Pet’s actual name, full background and so forth. As one should expect in dealing with Ninjas, this only yields an elaborate set of lies, a number of flawless fake identities and numerous dead ends.