The Pale Rider is the fourth and final Horsemen of the Apocalypse, according to Christian scripture. The rider traditionally represents death. Despite typically being portrayed carrying a scythe as his weapon (much like the Grim Reaper), in the Book Of Revelation, he is described as bringing Hades (Hell) wherever he rides as his symbol. His color being pale, as opposed to an actual color, could be reflecting the sickly appearance of a corpse (though old translations could be interpreted as the rider being pale green and yellowish green.)
"When the Lamb broke the fourth seal, I heard the voice of the fourth living creature saying, “Come.” I looked, and behold, an ashen horse; and he who sat on it had the name Death; and Hades was following with him. Authority was given to them over a fourth of the earth, to kill with sword and with famine and with pestilence and by the wild beasts of the earth."
"Origin: Middle East. One of the 4 Horsemen of the Apocalypse. He is Death incarnate, riding a pale horse. Some sources say the Four Horsemen will kill 1/4 of the world's population."
—Shin Megami Tensei A-Mode DDS Dictionary
The Pale Rider is one of three rare and difficult fiend bosses that can be found hidden throughout the game. If you beat him, he has a slim chance of dropping the Angel's Trumpet, the most powerful Law-aligned weapon in the game.
"One of the four horsemen of the apocalypse. He rides on a pale horse, and hades follows behind it."
—Shin Megami Tensei III: Nocturne Compendium
Pale Rider appears as one of the Candelabrum holders sent by Lucifer to test the Demi-Fiend, possessing the Candelabrum of Majesty. He first appears with the other Horsemen of the Apocalypse when the Demi-Fiend first ventures into the Third Kalpa. After Black Rider is defeated, he will appear in Asakusa, near the Collector Manikin's shop. Defeating Pale Rider in battle, the player will receive the Candelabrum of Majesty.
After being defeated and once the player reaches level 63, Pale Rider can be fused in the Cathedral of Shadows on the New Kagutsuchi phase if the player possesses a Deathstone, a special item found only in the Labyrinth of Amala. Pale Rider can be fused by any combination of demons resulting in a Tyrant Race demon.
Pale Rider appears as the summon orb boss of Shibuya's Celu Tower silver-level instance. He also appears as a boss within the Chain of Curse Deep Docks instance and a boss of the TR-Route of Denshi Kairo, through which the ability to fuse him can be obtained. He is a special triple fusion of Black Rider, Loa, and Bishamonten.
"So... a son of man hath called me from one underworld to another... I am the Pale Rider, one of the Four Horsemen. The end times approacheth..."
—When summoned through Demon Fusion
Pale Rider returns as a Fiend demon, however, before he can be summoned he must be engaged in combat in Sector Grus and defeated. Fusing Pale Rider with the other three Riders on a Special Fusion will result in Mother Harlot.
Pale Rider has a 1/256 chance appearing in Minami Sunamachi, on the western edge of the east-most pair of buildings, just south of the line that runs down the center lane (see below). His Almighty-spam mode is set by Force resistance. The reward for defeating him is a large amount of experience points, 3 Heavy Grimoires, and the ability to create him in a special fusion.
Pale Rider is fought alongside the other three in the Challenge Quest, Errand for the Apocalypse. Belial has sealed the four in a Domain found in Infernal Tokyo's Ikebukuro. They are weaker than when they are encountered normally. If Flynn makes any attempt to challenge them before accepting the quest, the Fiends will eject him from the domain. Completing the quest grants Flynn the Peacemaker, which is only surpassed by the plasma gun in terms of raw power.
Pale Rider, along with all the other Fiends, is encountered in Twisted Tokyo. Unlike previous games, they are not secret bosses and do not require tediously searching and resetting to encounter. Each Fiend starts with 3 Press Turns and gains one additional turn for every 2 Twisted Tokyo cycles, eg 4 turns on TT3, 5 turns on TT5, etc, in addition to increasing in level by ~75 levels each cycle; completing the first run and reaching higher cycles of Twisted Tokyo requires the "Eternal Torus" item from the "Depths of Twisted Tokyo" DLC ($1.49), and while not technically required the "Level Cap Unlock" DLC (free) is also needed due to the high enemy levels. Fiends must be defeated within 10 turns or Nanashi gets evicted to the dungeon's entrance, however the Fiend's HP does not recover when this happens. The Fiends drop a number of items equal to the current Twisted Tokyo cycle, up to 6 items at Twisted Tokyo 6; each has the possibility of dropping Incense, Armor Boost Kits, Strategems, Compendium Tickets, and a piece of unique equipment. Pale Rider is encountered on B9F and drops the Reaping Blade, and is fought at level 970 on cycle 9.
You can fuse Mithra and Ares to get Pale Rider. Although, before you can fuse them, you must complete Elizabeth's Request to fuse Valkyrie with Tarukaja. Then, she will award you with the Torn Black Cloth item, which is required for the fusion of Pale Rider.