Grinning Helper

Every sunrise changes it. From the second day it stops helping, backs away whenever you come within 10 blocks and follows from up to 24 blocks away, staring at you; anything it was still carrying is left on the ground. From the third day, each Minecraft block you place within 32 blocks of it (not a block from another mod) is copied 2 seconds later beside it, turned a quarter and leaning over as it rises, up to 48 copies a day. From the fourth night, while you are under a roof with a door within 8 blocks, it appears outside that door after dark, knocks, and stands there for 10 seconds, every 30 seconds. From the fifth night it stalks you after dark, moving only while you look away. When it reaches you, you are left in darkness for 6 seconds and it reappears 20 blocks behind you, keeping its distance for 90 seconds.

It never hurts you and never opens or breaks a door. A copied block crumbles into nothing when broken or blown up, so its builds give no blocks back. It has 20 health, doesn't breed or appear in the wild, and the only way to be rid of it is to defeat it.