Given the choice, I'd rather have lost my mind than this...
Headless Defender is a miniboss and a Spirit.
Acquisition
Headless Defender may be found second to the right of the "Collapsed Shack" Respite. To get to him, enter the first building on that block and climb over its right wall.
Battle
During combat, he swings his sword or thrusts it at close range.
If Lily is far away and within medium range, he can jump back to quickly charge up before dash forwards, whirling his sword and unleashing a flurry with extended reach.
In close range, he may slam his sword into the ground to release a shockwave that extends both ahead and behind him, covering a large area. This attack is charged up briefly as well.
On NG+, this ground slam attack covers a much bigger area, up to the entire battle field if he is near its center.
Spirit
When summoned, the Spirit appears near Lily and protects her with his shield. If within a short amount of time an attack hits the Spirit, even from behind, he will negate all its effects and swing his sword, dealing 275% damage.
Starting at the fourth level, the Spirit will also dash forward as he performs the swing, covering roughly 50% more area.
At the sixth level, his singular swing hits four times, for 20%, 40%, 55%, 160% damage (275% total). Extra range gained from the fourth level will be doubled.