When you deactivate your account, most of the data will be permanently deleted but some will still remain. For example, your videos will remain on Niconico and they will not be removed automatically. You need to delete videos before you deactivate your account.

Things that will remain

• Your videos, images, and Nico Commons uploads and comments.
• Your comments on videos
• Your My List comments
• Number of views/comments /My List of the videos you've watched
• Number of members in your communities
• Stamp Ranking will still be shown on the page after deactivating your account
• Stamp Page(deactivating your account will remove your nickname and link to your profile from the page)
• Play history and game accounts on Niconico App
  *Depends on the app but usually not be deleted immediately

Things that will be deleted or be hidden

• Your Profile (http://www.nicovideo.jp/user/account/ID)
• Nico Reports
• Users you follow
  (Your account information will be deleted from other users' pages after a certain period of time)
• My Lists
• Community ownership(It gets deleted after 30 days)
  *You cannot give the ownership after deactivating your account. Be sure to give it to others before deactivating your account.
• Channels you follow (Make sure to unsubscribe from paid channels before deleting your account)
• Unused Niconico Points and purchased/paid content