Filled with carelessness with 〜だらけ
Grammar Explanation: Covered in with 〜だらけ

Realizing Urushi is hiding in a box to try and scare him, Ayumu sits with his back to the box.
- 歩:
- 「どうですか?油断
( だらけのこの背中( …」- “How is it? This back filled with carelessness.”
Key Points
油断
( だらけ means full of openings〜だらけ attaches to a noun and means covered in or full of something undesirable.
油断
( is carelessness or having one’s guard down, so Ayumu presents his back as deliberately full of vulnerability.
油断
( だらけの modifies この背中( The structure is [noun + だらけ] の [noun].
Here 油断
( だらけの describes この背中( , “this back full of careless openings”.
どうですか is bait in polite form