Ohhh boiii I just wanna start off by saying that I absolutely love both Vidar and Sundew and would love to possibly toss Caspian at them if you're interested!
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Caspian was raised by his mother without the protection of a pack because he was born a bastard, so he has never had a fatherly relation with anyone. And, while his mother did her best to raise him properly, the fact that they were always alone up until his arrival here meant that his knowledge and perception of the world is extremely one-sided. He's slow to trust, and extremely unwilling to leave his comfort zone, especially when he is unsure of what lies outside it. All he knows is that he wants to return to the world he came from because he wishes to continue searching for his father, and he's going to be hot-headed and moody until he gets what he wants. But, I feel that Vidar can really temper him and help him mature into a well-rounded adult by acting as a pseudo father figure if they meet. The two of them are similar in that they've both lost everything, and they could help each other with their grief and the process of moving on. Maybe Vidar can help Caspian realize that not having known his father was not the end of the world, and that there wasn't any point in returning to the world he came from as there was nothing waiting for him anyways. And Caspian could return the favor by helping Vidar make peace with the loss of his mate, and reminding him that there are other things to live for, and that she would have wanted him to live a full life.
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On the other hand, I would really love for Caspian and Sundew to meet eventually because he's currently in the process of an angst-riddled, absolutely toxic relationship with Neith where he's going to exit it at the lowest point in his life. He's going to be disgusted with himself and his powers, thinking that he isn't worth happiness or love because he's a monster, and I feel like it would be really interesting if Sundew decided that she wasn't going to stand for that and worked to cheer him up with any means possible. She's all about positivity, and Caspian definitely needs some in his life. At the same time, he could remind Sundew that, for all she loves life and what it throws at her, not everyone is worth saving. And while he'll definitely use himself as an example for the most part, his point still stands even if he opens up lmao If Sundew succeeds and sticks around long enough to change Caspian's opinion of himself and the world around him, she'll definitely have found a loyal friend for life. Just as she put in the effort to piece him back together, so will he make sure that she never has to suffer what he went through.
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Also, I noticed that you mentioned that Sundew is prone to catching feels quickly, and if you're interested, I was thinking that it might make for neat threading to have her and Caspian interact in a romantic manner. Because of how things ended with Neith, he will definitely be against love for the foreseeable future in order to protect himself, but if Sundew gets him to open up, it'll only be a matter of time before he falls for her. After all, she's bright and cheery and everything he's ever wanted to be but never had the chance to because he was exposed to the ugly side of reality far too young. And, even if his feelings aren't reciprocated in the long run, he'll still do his best to take care of her until she finds the right partner for her. And, even then, he might not stop loving her, but he'll prioritize her joy above all else.
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Anyways! Those are just some ideas I had. I'm super sorry about the text wall yikies but let me know what you think!! I'm down for all plot ideas, so lay it on me fam! <3