Just recognised the art, stalked the artist's pixiv and realise it's the person who made Kanamewo, the short about a girl and a tree spirit (confirmed it by identifying art that was used in other videos of same youtube account)
They sure like bonds between people/girls with a tragic setting/ending

Thanks for mentioning it, that was a really good short which seemed like it could have come straight from a Ghibli movie. I wonder if the spirit was still alive, in some way, at the end of the movie.

The sempai is the same one that Kase helped in chapter six so I am pretty sure that they never dated and she was just helping her with her training and we're friends, but I could be wrong.

It takes a few minutes to get used to the format, but otherwise it seems quite interesting.