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Tested only briefly, I don't see anything to suggest it isn't fully portable and doesn't add any data in %appdata% or my documents

manuskript - http://www.theologeek.ch/manuskript/
Downloads & Instructions https://github.com/olivierkes/manuskript/releases

(Similar category to Celtx https://www.portablefreeware.com/index.php?id=1349 - but manuskript is free and still being developed, while Celtx is now quite old and newer version are subscription based it seems)

Manuskript provides a rich environment to help writers create their first draft and then further refine and edit their masterpiece.
With Manuskript you can:

• Grow your premise from one sentence, to a paragraph, to a full summary
• Create characters
• Conceive plots
• Construct outlines (Outline mode and/or Index cards)
• Write with focus (Distraction free mode)
• Build worlds
• Track items
• Edit and re-organize chapters and scenes
• View Story line
• Compose with fiction or non-fiction templates and writing modes
• Import and export document formats such as HTML, ePub, OpenDocument, DocX, and more

Additionally Manuskript can help in many more ways with a spell checker, markdown highlighter, frequency analyzer, and automatic save in open and plain text file format.

Screenshot

FYI, I was afraid Manuskript was yet another Elecron-based app but after checking I discovered it is in fact Qt-based...

Update: now at v.0.15.0 as of about 6 months ago

https://www.theologeek.ch/manuskript/category/news/