The Liko vs. Spinel contrast is interesting in hindsight because Liko's strength in her kindness, empathy, and capacity for connection and understanding feelings has even in her early days unintentionally set her up from Day 1 to be the biggest threat to Spinel's mindset and worldview, not just his ambitions.
Spinel's whole thing underneath his outwardly nasty actions is thinking primarily in terms of facts and logic, meticulously arranging things to go a certain way based on assessments of all variables, and being 100 steps ahead, but he doesn't account for emotion and connection and how it can flip the script against him in surprising ways.
His first plan to steal Liko's pendant was to try to bait her into an alleyway using her kindhearted nature against her, then temporarily wipe her memory, incapacitate her, and swipe the pendant and escape without leaving a trace, while using the time she spends wandering around and misdirecting Dot through fake online posts to stall the RVT while he escapes without any trace of his existence of proof that he was around. He knew the memory wipe was temporary, but in his eyes, it would buy him enough time that once she did regain her memory, it would be too late to catch up to him and he'd be back at the base with the pendant in hand. What he didn't account for was Dot, not just in her ability to use her clout as Nidothing to her advantage, but the fact that Liko for the past several episodes before that went out of her way to befriend her and try to reach out to her was something Dot was grateful for. She decided to repay that kindness by going out of her way to find her using her clout and connections to her advantage and also fighting for her out of sheer loyalty. Dot was the unknown variable because of Liko specifically, where things Liko did for her led to her getting involved out of emotional investment and successfully backing him into a corner.
His other plan to get Amethio suspended from the Explorers involved trapping him specifically with Liko together in a cave, and he deliberately chose a cave that had a small opening to above ground, and had Beheeyem spy on that spot to record footage of Amethio escaping together with Liko. He used Amethio's more honorable nature against him and knew both that Amethio was smart enough to figure out how to escape and that he would not hesitate to help someone when necessary, even if said person is an adversary. What he didn't account for was the fact that Liko would actively take the time they were in that cave together to try to connect with Amethio and understand him. So even though he successfully managed to frame Amethio, he didn't account for the fact that it led to him and Liko forming a genuine connection and bond, and even though Amethio never formally joined her, he's been willing to ally with her when necessary ever since. And it's obvious from the way he talks about them, even in the recent episodes, that the fact that Amethio and Liko have an alliance is something he feels genuinely threatened by.
She was also the big factor once again post-timeskip when Spinel doubled down to mentally crush Amethio and force him to give up, knowing that he was tough enough to stay alive but by relentlessly pursuing him and forcing him into isolation he'd resign himself and give up trying to fight. Liko was the biggest factor in not only finding him but him coming back and being ready to fight again, much more than any of Zirc, Onia, or Hamber were. Sure, the former two are unwaveringly loyal to him, and Hamber is also his surrogate parent figure, and they went out of their way for him, but they alone could only do so much. It was when Liko got involved, thanks to Zirc and Onia running into her and knowing how much Amethio respected her, and in turn asking her to help them find him, that made the big difference. Liko is very attuned to emotion and feelings, and she figured out where he was through the family photos in his mansion, which was something Zirc and Onia couldn't figure out by themselves (no disrespect to them).
So yeah, out of everyone, the biggest threat to his methods, mindset, and ambitions is basically Liko, and in a way, always has been her. For her to be the one to confront and battle Spinel directly is super fitting in that regard.
