In the app, you should be able to delete your account (settings or the like). If that doesn’t work however, contact the Line makers using: https://line.naver.jp/cs/en/
Fill in the appropriate fields and explain your story/request. In their ToS/PP it is clearly stated that closure of your account/data is a true deletion. No turning back!
URLS:
https://line.naver.jp/
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.