I’ve heard ppl say “How to Tame a Knight” has the same author as crimson ocean and it seems of a similar vein, though not fully translated yet. I’d also recommend “PREDATOR” (the one that’s all caps on this site) since it has a similar “relationship with an alien” plot
The MC can see ghosts and this one ghost helps her but she goes to try to protect the second prince from dying and she becomes his maid/informant ┗( T﹏T )┛
Why did I look for Harem and looked in the popularity section and I seen the manga of Naruto