Why do you think Marcel idolizes Marissa so much? He values subtly so much more than she does, in fact he seems to find passion to be a major character flaw.
While I headcanon them as twins, canonically speaking we know he is the younger sibling (although it’s not more than 4 years of difference between them), so I think Marisa was very much a protector to him. Not just out of kindness, cause she was often very self-serving, but I think that by protecting him from their mother, Marisa gained his loyalty and devotion. We can’t confirm what happened through their lives, such as the affair event, but having Marcel’s loyalty sounds like something useful. I imagine she made full use of his position and influence, even if it was little at first. Marisa, to me, loves people according to how useful they strike her, how they affect her life in a practical level.
I also think that because they were raised by a narcisist, both of them had a hard time connecting with other people in meaningful ways (and this will tie itself with another meta ask I got, about Asriel), Marcel more so than most. We have very little to analyse in the book, just a single scene between him and his mother, but to me that’s very telling about how they might have been raised. He is definitely the neglected child, even if Maman was harsh and abusive towards them both; Marisa strikes me as the golden child, so she took the worst of maman’s abuse because she was always in focus. However, Maman’s attention, whenever turned to Marcel, was damaging and aggressive, so he needed something to cling onto and that something was his sister. They both understood what it was like to endure their mother, and they both loved her in a conflicting way, and they both want nothing to do with her (Marisa moved away, or it is implied she did by going to England, while Marcel treats Maman very poorly, yet he stills has her under care when he could simply abandon her).
I also think that Marcel idolises Marisa because that’s the scenario he grew up on, hearing his mother praise his sister and diminish him. She does that even after Marisa’s vanishing, in TSC. Questioning his subtle ways, his masculinity, his lack of resolve (which I personally I think she’s dumb, because Marcel is as willful as Marisa, he’s just quiet about it). So, his whole life has been a series of “Why aren’t you pleasant like Marisa?”, “Marisa was all the man you’ll never be” and so on, and he heeds those words for the most part, and I believe he wishes he had Marisa’s strong traits. She was very accomplished in a short span of time, and well-known, while he is mostly a dull employee and only truly becomes a force to be reckoneded with in the late stages of the Magisterium Congress.
Like you pointed out, they are very different from each other in many ways. Marcel is more subtle, more cautious, but he yields better results in my opinion; Marisa is very ruthless and commanding and she gets faster results, but often with a lot of collateral damage. Marcel is a long term type of man, while Marisa is a immediate results type of woman (very much like her mother, when we think back to that scene, where Maman. Marcel also has very little friends and his social life is very dull and scarce, while Marisa is a social creature, thriving best among people because she is, at her core, a people pleaser (?). She can charm most people, so her ideal scenario is to be involved in social situations, while Marcel is quiet and keeps a very formal relationship with people, as far as we’re told.
I do think that they wouldn’t always agree with each other, especially their methods, and I don’t think there was sweetness between them - they’re both very emotionally stunted (?) people, but there is a sense of cumplicity between them, which comes from them being family. (And this is why I think that, if my reading of his character is right and Philman doesnt fuck up lol, Marcel wouldn’t hurt Lyra because she is something that mattered to Marisa. Anyway, I am with my clown hat on lmao)