Fun fact: Bishamonten in Buddhism, is a Guardian to Indra.

So, as most people would be well aware the Highest Realm in Buddhism that is still part of the Realm of Desire is that ruled by the Devas, this, however, is only the first part, as the Deva Loka is divided in different parts that relate to how close one finds themselves to Nirvana and therefore to finally attaining enlightenment.

The Kāma-Dhātu-Deva are the Six Heavens that form the Heavenly Realm and the further one finds themselves within such realm, the closer they are to liberation, the lowest of there is the more one is controlled by pleasure and desires, furthermore your lifespan is larger the further you find yourself inside the land.

The Four Heavenly Kings, Dhṛtarāṣṭra (East), Virūḍhaka (South), Virūpākṣa (West) and Vaiśravaṇa (North, and also Bishamonten's Sanskrit name) live in the mountainside of Mount Sumeru, at the lowest of the Heavenly Realms, where desire still exists, this is the case in all of this Six Heavens and ceases to be once an individual reaches the Form Realm where Brahma and others reside.

Because they are in the Heavenly Realm their day and night are equivalent to 50 years in the human world, and their lifespan is fixed at 500 years, which means that, in human terms, their lifespan 9,125,000 years, which is indeed a lengthy lifespan though very short compared to those in higher places. The Four Heavenly Kings are also, and here is the main thing, the Guardians of the King of the Devas, Indra.

Speaking of Indra he and the Hindu Gods live in Trayastriṃś which is alocated atop Mount Sumeru with Indra living in Sudarshana which is its central region, their day and night are equivalent to 100 years each and their lifespan is 1000 years, meaning that in human years the Devas reach a total of 36,500,000 years.

Beyond this are the Heavens that have left the earth, because you'll see it is understood that the Four Heavenly Kings and the Devas are living in the Heavens but also because they live in Mount Sumeru and not above it they are still in Earth, Mount Sumeru is understood to be 84000 Yojanas high, or 1,680,000 kilometers, but above it are Heavens that reside in the skies themselves, and its beings are not involved in conflicts with the Asuras, for example.

These are the Yama Heaven which is associated not with the Great King Yama that Wukong is an enemy of but with Yama the Heavenly King, which is generally understood to be the other's twin, with one Yama that carries punishment in Hell and one that carries Heavenly duties above the skies. Their lifespans last 146,000,000 years.

After this is Tusita, where Maitreya is currently awaiting to descend to the world of the living as the next Buddha after Shakyamuni. Those who are born here have a lifespan of 584,000,000 years.

Next is Nimmānarati which is the second highest realm and is named that way because it allows people to enjoy and transform themselves within the realm of the five desires. The lifespans here last 2,336,000,000 years.

Finally Hajun or Mara livea right above this place in Para-nirmita-vaśa-vartin, it may sound weird but yes, Hajun who was Buddha's real enemy lives at the highest place that is still part of the Desire Realm, far above the Gods and and even above a Bodhisattva in Maitreya, in this land the lifespan lasts 9,344,000,000 years and it is ruled by Hajun.

If you wish to know there is an exception within the Indian Gods to this, which is in Brahma, Brahma forms part of the Form Realm, which is the Realm above Para-nirmita-vaśa-vartin, in that realm beings have no desires or pleasures but have physical bodies, that realm is divided in four and Brahma rules the first.

So, long story short, Bishamonten is a Guardian to the God Indra, who is the ruler of the Second of Six Heavens that are still both physical as well as under the effects of desires. Makes me wonder if they knew each other in the manga.