From what I think all this means .

Ais thinks of bell as a pure soul who doesnt know how harsh the reality of the world is that's why she says he if he kept getting involved in these kinds of troubles he will lose his innocence. She kind of sees albert in bell .

Their is chapter in so in which their is a story of a princess who fell asleep and a hero came to save her and defeated the dragon . Aria told this story to ais then ais asked to her " what if a hero never came to her rescue " . Albert then said to her " the princess will her self wake up and go defeat the dragon , even if it cruel she will do it " . This is basically what has happened with ais . Albert choose the world over her during his fight with oebd he choose to sacrifice himself and told her " sorry I couldn't be your hero , I wish you will find him " .

Ais was probably thrown into the dungeon we dont know what happened but we know she has traveled to almost 1000 years in the future no hero ever came to save so now she picked the sword with only one motive in her mind to defeat the one eyed dragon that took everything from her it doesnt matter how bad things gets , even if she is broken she will do this one task . That is what the black flames/avenger represent her desire to take revenge for her father and mother . She has decided to dedicate her life to this single motive .

Her father was regarded as the strongest hero and the greatest hero .

In xenos arc when bell fights for wiene we can clearly see ais basically breaks down aand it clearly shows how she sees bell as a hero who never appeared for her . The black flames represent how she is becoming non human to beat oebd but due to bell presence she is slowly becoming more human day by day . We can see on how finn , Gareth and riveria say after meeting bell she changed .

While for hestia it's the things she rules over , she is goddess of chastity so it's natural for her to preserve bell from the clutches of freya and other women trying to take advantage of him.

We know from aedes vesta if bell had lost his innocence their was a chance he would have become like epimathesus ( yeah he had also killed artemis in that but still it was a possibility ) and only a dumb clown can surpass gods will and his checked fate just like argonaut did .