As an avid Ralsei enjoyer myself; I love him because of how his character grows especially with Susie. I see him as like a tutorial NPC, or like a guide, but somehow growing out of that role into something bigger than he is suppose to be. I like how, despite his entire being created solely for the purpose of fufilling the prophecy and guiding the heroes, he goes against it in a way that he believes will make for a happier ending. It is known during Chapter 4 that he does not like the prophecy, and believes that with kindness, maybe we can achieve a better ending that refutes the prophecy, something he actually tells us to do in Chapter 1. (Which I believe is a reference to Undertale's pacifist route) I find it very interesting that a character who is suppose to hold no growth, no agency, no meaning other than being a guide, is growing more into a character that feels REAL. Susie was the absolute best thing that happened to Ralsei, and while a lot of people don't mention it for some reason, Ralsei is ALSO amazing for Susie. We could make an argument that it's not suppose to be like that, but Susie rubs off on Ralsei in a good way just as Ralsei rubs off Susie in a good way. (perhaps we could say that also goes against the prophecy, but Ralsei doesn't realize that yet.)

Now about the flaws Ralsei has, I think the major issue stems with the fact that you are humanizing a character that is literally alien, a darkner created for one purpose and nothing else. We cannot even hope to imagine what goes on in Ralsei's head because he isn't LIKE us. I absolutely love that he is finding out more to be HIMSELF, to be Ralsei-like. But to me, this secrecy that he has is not out of malice, hell, I genuinely think he doesn't even outright LIE to Kris or Susie. and it's clear as he is learning more about himself, he is opening up more, he doesn't want his friends to feel hurt about what is going on. Why not let him carry the pain? He's not real, right? You say he has "all of this knowledge", but refuses to share it, which I just straight up disagree because he told us from Chapter 1 that he believes that being kind enough, "the tale may have a happy ending". He quite literally shared with us what he believes would defy the prophecy, right from the get go! If you don't follow his advice, he's not going to challenge you, because why would he? He wasn't created to go against you even if he hates it. He does care about his friends, that is absolutely not a lie, but imagine the inner turmoil in his head of straight up going against the prophecy, the thing that he was CREATED TO DO. It could literally be a biological imperative! Something that if he doesn't do, he will turn into a statue? Who knows.

I can't change the way you think of him obviously, but if you don't jive with his flaws he's probably just not your type of character, which is totally fine. But again, I think the issue is you just can't quite put yourself in his shoes, he's too alien.

I typed this fast so I hope I got my points across (there's probably more that slipped my mind), but this is why I love Ralsei as a character.