Yomotsu-Ikusa were the demonic beings that lived in the Shinto underworld, Yomi. Yomotsu-Ikusa are employed and led by the guardian of Yomi, Yomotsu-Shikome, as a vast army. They are also commanded as troops by the gods of yogi.
In Japanese mythology, Yomotsu-Ikusa appear as the troops sent by Izanami to pursue Izanagi when he fled Yomi after seeing Izanami's face after her death. The number of Yomotsu-Ikusa sent is said to be 1,500. Just as they are about to overtake Izanagi at Yomitsuhirazaka, he drove them back by throwing a peach, believed to have demon-repelling powers. Izanagi was thus able to escape.
Yomotsu-Ikusa can be contracted in the new moon instance of Shinagawa Catacombs of the Templar Knight bronze-level. They appear as enemies in Ikebukuro as demons contracted by the punks and in Old Ichigaya Camp's B108 summon orb boss room alongside Tokisada.
Shin Megami Tensei IV
Yomotsu Ikusa is summoned by Hiro Jingu in the third round of the "Hunter Tournament Finals" Challenge Quest. Yomotsu Ikusa can be encountered during the test at Tsukiji Hongwanji, but any attempt to scout them will result in the demon blowing on the candle Flynn must keep lit. Yomotsu Ikusa can be evolved from Shikome once she reaches level 31. This demon can teach Flynn the Rapid Needle, Fang Breaker and Tathlum Shot skills through its Demon Whisper.
Shin Megami Tensei IV Apocalypse
Yomotsu Ikusa can be found in most areas of Ginza. It can teach the Fang Breaker, Tathlum Shot and Blast Arrow skills through its Demon Whisper. Yomotus Ikusa benefits from learning Gun and Physical skills.
Devil Summoner: Soul Hackers
"Demons who serve in the army of the gods of Yomi. Izanagi chased after his dead wife to the land of Yomi, but he broke the taboo and saw Izanami's unsightly face. He started to run, which angered Izanami, causing her to send the Yomotsu Ikusa after him. Just as they nearly caught him, Izanagi threw a purifying peach seed and escaped."
—Devil Summoner: Soul Hackers 3DS compendium
Devil Summoner: Raidou Kuzunoha vs. The Soulless Army
"Izanami sent them after Izanagi at Yomotsu Hirasaka, the entrance of Yomi, the Japanese underworld. The army of Ikusa is led by Yomotsu Shikome."
—Devil Summoner: Raidou Kuzunoha vs. The Soulless Army devil chart
Can be obtained from Shuffle Time in Marukyu Striptease. This is the only Persona that when called out (hitting a weakness or critical hit), does not have its full name read out; instead, the protagonist only says "Ikusa!"
Devil Survivor Overclocked
He appears on Day 4 as an optional boss labeled Netherworld Soldier. He is in Aoyama Cemetery, amassing an army of the dead, ordering them to go into town and avenge themselves. The protagonist and his friends will try and stop them, but right before the fight a man will appear and accuse them of summoning the demons, ensuring they can't let any of the undead invaders escape, making the ensuing fight much harder. Ikusa will summon Loas, Legions and Pyro Jacks to fight; some try to fight, but most will try to get around to the exit; this will cause the party to lose the fight. The boss and his demons are quite overpowered and should be attacked on a second playthrough. It's also recommended that the party saves Midori in order to ease the burden of the fight with a fourth team.
Shin Megami Tensei: Liberation Dx2
"A demon soldier in the Yomi army from Japanese lore. He loyally serves the gods of Yomi no kuni. When Izanagi tried to rescue his wife Izanami from Yomi, Ikusa was one of the monsters sent to chase after Izanagi when Izanami had become a hideous hag."