In the lands of MAR Heaven many years after Babbo was sealed for the third time there was an incident that caused all ARMs to vanish they then appear on Earth most of them sold as trinkets as they would not activate as if their powers were sealed and noone can sense their power. (well almost no one Azazel sensed a glimpse their power when he decided to research them as they appeared our of no where not that it helps that one appeared in his lab specifically the ring weapon ARM fishing Rod [to give a reason to his sudden intrest in fishing)
When Issei ran from home after finding out Irina moved away he ran into the forest not knowing where he was going there he wanders off to a cave where he finds a chest inside he finds a strange metal kodoma (Babbo can change to a smaller version the hammer stays the same but the ball loses its face when its smaller) he takes it as a lucky charm that he later always takes with him
Babbo awakens the morning after Issei becomes a devil cause of his lack of magic and with Babbo's awakening it reactivates the ability to use the ARM when Issei wakes up he freaks out but then just goes with it (also when in his smaller form Babbo can talk to Issei telepathically)
Issei will need to gain more power to use more of the magic stones on Babbo feel free to create and 7 forms that Babbo can take but keep in mind Issei will need to be strong enough to use it he also stills have Ddraig during the training trip Issei makes Babbo's first form a clawed gauntlet that looks like the Boosted gear but with a retractable blade on it (so he doesnt have to just punch with his boosted gear hand)
Issei isnt a pervert as he didnt meet the old man
Koneko wears snowman doll as a pendent from Kuroka cause her snow white hair its the guardian ARM Snowman
Kiba wears Necklace Blade he bought it one day as it caught his interest (said to have dark powers but it never shown you can make it up)
Akeno wears Electric Feather (its a ring) as a memento from her mother
Asia wears the holy ARM Healing Angel (pendent) which Issei buys for her not knowing it to be an ARM when they go on their date
Roseweisse wears a ring thats the guardian ARM Yggdrasil
Xenovia wears a bangle from her mother thats the guardian ARM Saint Anger
you can give others people ARMs too even make up some of your own too but keep in mind this
Simple ÄRMs are little more than simple tools when active.
Holy ÄRMs possess the power to heal and undo curses inflicted by Darkness ÄRMs. Known in the dub as Sacred ÄRMs.
Weapon ÄRMs are used as weapons in battle.
Darkness ÄRMs are powerful artifacts typically used to curse people usually at a cost to the user.
Dimension ÄRMs can teleport anyone to different places, create pocket dimensions, and occasionally allow someone to watch scenes happening far away.
Guardian ÄRMs summon a monster or a 'guardian' of some sort to protect the owner of the ÄRM. The drawback is that as long as the guardian is active the ÄRM user is unable to move from the spot he was when he summoned it.
Nature ÄRMs are used for certain elemental attacks, or enhancing a body's physical performance.
Ghost ÄRMs attach to the user's body and allows the user to use his or her body as a living weapon.