Can a story be mainly driven by monologues? Yes, it can.

Ellen, a girl so sick she can't walk, a girl so bound to her bed she can't experience the outside world and more importantly: a girl who nobody loves.
Ellen was sick from the day she can remember, she can't wash herself, she can barely walk, she can't go outside, she can't find friends, as a consequence of this she only knows her parents, her mother takes care of her, loves her,while her father can't even look at her, he hates her because of how she is and even more the day Ellen's mother leaves him for another men. The day she comes back Ellen kills her, she waited so long for the only person who loves her to come back to her but had to realize that her mother left her, hates her, is tired of taking care of her and her father couldn't care less about Ellen so she kills him too. As she runs out of the house she nearly dies because of her body being as sick as it is but before she dies she meets a black cat who offers to save her, if she agrees to becoming a witch.
When she wakes up she is in a mansion, seemingly cured, greeted by the black cat who tells her that she won't be sick anymore as long as she doesn't step outside of the house, but after having passed enough tiime walking through the mansion, discovering the magic it inhabits and reading all the books she can find she carelessly steps outside where she gets haunted by her sickness again.
From there on her quest to finding a cure and trying to finally be loved starts, the manga passes most of the words expressing her feelings and thoughts, she thinks about the outside world, she thinks about her parents, about the inside of the mansion, about the people who visit her, about how much she craves to finally be cured so she can be loved, about how little she likes the cat, about her time as a sick girl, about the people who want to kill her, about her friend, about how long she's been in the mansion, how people outside of it think of her and about her future.
I enjoyed this work a lot because it shows how much Ellen craves something and to what lengths she will go, how easily she'll strip herself of her humanity in an attempt to finally be with other humans, she doesn't get a big drop into insanity, things just happen and she does what she thinks she has to, she doesn't enjoy it but she also doesn't really dislike it, she just does it and that's it.
Her relationship to Viola ended in such a perfect manner because it showed how one can feel horrible things after caring for someone, it showed how little Ellen cared about Viola and how much she even hated her, most of those things expressed either through monologues or through short talks with the cat.
Truly a great read, the only thing that stopped me from giving it a 10/10 was that after the boy came back I wished for a little more of a reminder of how she was seen by him, similar to the talk between the cat and Viola, otherwise I had nothing at all to complain about, the art was great, I like Ellen a lot, Viola was great, the end even better and I look forward to more such stories.