She is described in her bio as being born from the hope of Zaunites and sailors similar to Evelynn who grew from pain. She might be a demon who figured out helping people was the best way to allow her to feed from their hope.
Demon is just a word used to describe evil spirits according to Riot, Janna is a wind spirit, but not evil.
Runeterra is not our world, and it has nothing to do with any real world religions. Demon is really just handy way to say "malevolent spirit preying upon humanity"
Since she is not malevolent she is not a demon.
I like the angle. Though it is more likely she's an angel or fey or whatever the good version of a demon is in this world.
But the thought of a manipulative demon spreading hope and goodcheer because of dietary requirements sounds like a great story to write.
I think that's actually a pretty interesting question. Short answer as others have said, no she isn't she is a "good spirit". However as Ebon said, demons are a word for evil spirits.
So that could raise the question, could Janna ever become a demon due to evil influence/mind change? I highly doubt it, but its possible perhaps.
Janna works off of devotion similar to a God. The more people who believe in her and acknowledge her, the more powerful she becomes
???????????
Whats really amazing about janna is how shaved she is
From what I can tell from the league races page.
Janna is considered a demon because she feeds off hope.
Not all Demons are inherently evil.
Janna's a wind spirit sort-of goddess that gets her power from her following. The word "demon" is quite loose -- it's generally just used to describe evil spirits.
Janna is a spirit not a demon
The difference from the regular spirits and gods, like 
and 
, they exist because people believe in them and pray to them. Whereas demons such as 
and 
prey on humans physically drawing out certain emotions/ aspects of a person and feeding on it.
i kinda like the twist lol but instead of praying on humans, she does the opposite on helping them. turning her back into her primary goal, to kill humans.
i also like the her representing the good spirits as eve,tk and noc represents evil spirits.
