Epyonnn @MimiTea
Hum, simple sometimes is best. It does indeed show what the story is going to be about. Which is great. Furthermore, if he drew it himself it means it's his own image rights. Which is even better. As for an actual critic, here's my two cents.
To my opinion, it lacks a certain impact. A punch so to say.
That might be due to how cramp the cover is. It is good in a way since it forces people to focus on the important stuff. However, I don't sense this "wow it looks awesome!" vibe. Maybe because it lacks some background image. We only see a dark background with some kind of knight turning his back to the readers (which is cool in my opinion.)
Also, the lack of color might be one of those variables. It does indicate the tone of the book tho. So it's far from being a useless thing. I haven't been through your story yet, so I might come back to that afterward.
The cross is downright well done, as well as the highlighted letters. Clearly tells us what to expect. Although I think you should align those letters to make it look more professional. A neat/clean cover is more attractive in general. Ah! There is also that "Character" size. It looks compressed, I feel out of breath just looking at it. Try to even it out. You could, for example, reduce the size of the white letters but keep the red ones just as big as they are now.
As for those sakura petals in the background.... hum. It's weird. It feels like there are here just to be fillers. To fill in those empty areas.
Overall, it's not a back cover at all. It does what it's here for, but it does lack the "wow" vibe. But that's really hard to do, you have to order a cover from an artist to get that vibe. I think you can leave your cover picture as is, or maybe implement the "letter size" change I talked about.
Hope it helps.
As for the font...hmm, I'll come back to that after reading your story.