I feel like people are judging Canaria unfairly due to misconceptions constructed by other isekai series. Canaria seems to be something around 20 years Cesar's senior. I don't remember her age ever being explicitly stated, but it is clear from her look at the time of the accident that she is not in her teenage years. So, while the two in that world are the same age, she is in fact much older. Since there's such an age difference she has never seen him in a sexual manner. That's stated from the very start of the story. At the current moment in the story, it's been ten years since they met. Cesar is 22. Canaria is most likely at least 42 spiritually.
To put it into perspective: Drake is dating an 18-year-old model named Bella Harris. They've known each other longer than that. Since she was 16, in fact. But before she turned 18 they were 'just friends. To add to that, the actress that plays Eleven on Stranger things described her relationship (when she was thirteen) with the rapper like this:
"He a great friend and a great role model. We just texted each other the other day. He was like ‘I miss you so much,’ I was like, ‘I miss you more,’ he’s great.”
Now I'm not going to say that the age difference between Drake and Bella is similar to the one between Cesar and Canaria, because it's not. It's actually Drake and Eleven that have the similar ages. Drake was thirty when she was thirteen and that very aptly fits with the spiritually thirty-ish Canaria and twelve year old Cesar. It's just that Isekais have conditioned us that a forty year old reincarnator getting cozy with a sixteen year old isn't weird at all. It's basically the 'but she's actually 300-years-old, not twelve' of the Isekai genre.
To add to that, Canaria has known him closely for ten years since he was a child. Someone that is twenty years younger than she. I'm pretty sure if you were to describe her feelings towards him right now, they would be maternal, rather than amorous, and that would be normal.
I'm not saying she isn't dense, I'm saying that she's treating him like a kid and not realizing that his actions aren't just him acting out like he's done during the t e n y e a r s she's known him, but actual feelings of love. Especially given that she has no interest like that in him.