To trigger the Patch branch plot, you must first complete the plot related to Brother Onion in the Street of the Undead.

After defeating the two giants, Patch will have additional lines. Go through the attic and go in the opposite direction to see Patch taking off his armor.

Talk to him, choose "Don't act stupid", and get the action "Float on the ground to apologize". Talk to him again and get "Rusty Coin". You can then buy goods from him. After purchasing the Onion suit from Patch, you can return to the church to return the equipment to Onion Brother. This is one of the processes of Onion Brother's side plot.

Summary:

Purchase the key to enter the tower; see the contaminated fireproof female soul to gain skills; trigger Patch; return to the Fire Sacrifice.