A follower is a monster that follows you from one dungeon level to another, if it happens to be next to you when you change levels. Elbereth will prevent this if the monster respects it.

Monsters which follow you across staircases, level teleports, trapdoors etc. are:

• the @ Wizard of Yendor
• @ Croesus
• angry shopkeepers
• gremlins
• stalkers
• salamanders
• ghosts
• shades
• mercenaries (soldiers etc. and watchmen/captains)
• all quest nemeses
• all zombies (Z except ghoul, skeleton)
• all demons (i and &)
• trappers and lurkers (t)
• angelic beings (A)
• trolls (T)
• vampires (V)
• wraiths (W)
• pets

Monsters with the Amulet of Yendor won't follow across even if they would otherwise, and fleeing monsters will not follow, unless you're carrying the Amulet. Pets and the Wizard will follow even if they are fleeing.

Not all covetous monsters are followers. In particular, liches and named demons won't always follow you down stairs.

See levl_follower in mondata.c.