As they are classified as undead in Strange Notebook, spirit minions are proposed to be ghosts altered in some ways by an undead lord into his thralls to carry out his bidding. Many of them encountered by Akko and co. over the course of their exploration at the seven regions accessible via Sealed Door of Horologium Chamber. As with other monsters, they are shown to be aggressive, though it's not elaborated whether ones encountered by the young witches are still working for the undead lord whom converted them, had become independent undeads for quite some time and encountered by chance, or both. Hollows however, are mostly (if not all of them) independent spirit minions since they are not mentioned working for any undead lords unlike Invisible Assassins according to Strange Notebook. One of the Hollows even shown assisting Molly and the girls arranging books at the library following the sealing of Horologium Chamber by Ursula. Even so, it's not even elaborated whether the said Hollow was a truly independent spirit minion contracted by Molly as her familiar or rather, a magical puppet which strikingly resembling one controlled via. Puppet Magic.
There are two types of spirit minions:
—Description of Hollow from Strange Notebook
Hollows are a type of spirit minion that resembles a cartoonish ghost with a gray ghostly tail, large sleeves that covering their hands, and a hood that obscured their face save for a pair of glowing yellow eyes.
—Description of Invisible Assassin from Strange Notebook
Invisible Assassins resemble Hollows a bit, but with black body, exposed skeletal hands, and what appeared to be a bull skull as the head (though the said skull might actually some sort of mask both to conceal their face and showing their allegiance to undead lord who converted them).
Powers and Abilities
While not very powerful, Spirit Minions are still formidable adversaries. Both are capable of disappearing and reappering at will as well as projecting fiery purple bolts, but Invisible Assassins are much more dangerous thanks to their ability to ambush their victims and snatch away their soul, rendering them in vegetative state until their stolen soul is restored. As with regular ghosts, they are suspectible to anything that can banish them, including magic.