Spinel having an Umbreon as his main partner strikes me the more I think about it because it paints his character in a more interesting light than being simply "evil". Umbreon is a friendship evolution, evolving from Eevee through a really close bond with its Trainer, and Spinel's bond with Umbreon is completely genuine. Now it's easy to say that he's a sociopath who has been emotionally manipulating his Eevee/Umbreon, but I don't think that's the case. On the contrary, the bond between Spinel and Umbreon is mutual and genuine. Spinel genuinely loves his Umbreon. He cares about it and values its company. Umbreon in turn is also completely loyal to Spinel and devotedly protects him and cares about him. Spinel in turn appreciates its loyalty and it's a two-way street: Umbreon also communicates with Spinel and Spinel genuinely trusts and listens to Umbreon. Back when Arboliva came out to threaten Spinel, Umbreon urged him to retreat, and he listened. Their bond is strong and genuine, and they genuinely share a connection with each other where they understand each other at a deep level.
It stands out because what this suggests and even highlights about Spinel is that he is in fact, not actually a psychopath or sociopath, and is quite the opposite: he is actually functionally empathetic. He's not devoid of empathy like psychopaths and sociopaths are. On the contrary, he possesses a very high level of empathy. He's perhaps almost too empathetic even. That sounds ironic when you look at how he acts on a surface level, but it's very much true.
He is capable of connecting with others at a deep level and understanding them in a way. His bond with Umbreon, a Pokemon that evolves from Eevee through a deep bond with its Trainer, is a testament to that. But he's afraid of doing it because he's so full of empathy. Building connections with others, getting close to them, learning about them and getting to know them as people, it's all things he is capable of doing. But it also hurts him to do it. Because doing that means seeing everything about a person, not just the good things about them but also the bad and ugly things about them. And it also sets him up to be put through pain and hardship, because building connections means subjecting yourself to potential betrayal and misunderstandings. Humans are messy and complicated by nature. Sometimes people will betray your expectations, sometimes you misread other people's intentions. Sometimes even when you do your homework and keep your nose to the ground people will do things that don't work in your favor. And that scares Spinel deeply, because he's very emotionally sensitive and prone to being hurt as a result. Being hurt puts him through more pain than most. And he doesn't want others to get to know him because he's afraid of people hating him for what they know about him.
Spinel hates complexity. His underlying laziness, which I talked about some time ago, comes from a hatred of effort, optimism, selflessness, potential, and ironically, a hatred of lies. The fact that the world and people are complex and messy is something he hates, because it means the ugly side of people will hurt him, and he assumes the uglier sides of people are their true intentions, so he's afraid of getting close to people because of those sides.
His Umbreon's moveset consists of Snarl and Reflect, the former of which lowers Special Attack and the latter of which halves physical damage. It highlights his Umbreon's loyalty and how it's his devoted protector, but it also highlights Spinel's own tendency to keep people at arm's length and keep his guard up. He's guarded and assumes the worst of people, and thus pushes people away: Foul Play using the opponent's strength against them highlights that he sees the opponent's strength and tries to turn it on them to push them away, and overall his unwillingness to let other people get close to him and get to know him, only seeing the facade he's put up.
The word "facade" being especially important. It's actually quite meta that his outward behavior throughout the series is similar to that of a sadistic, malignant psychopath, because that's the "facade" he puts up to protect himself from others and keep them away from him. He's not actually that: his "psychopathy" is entirely a fake act. But he's putting on that false front because that's what he wants people to believe he is. He wants others to believe that he's literally entirely evil, so that they stop making any effort to get close to him and go away, and thus have no chance of ever hurting him, either by showing their ugly front or using what they know about him against him.
Deep down, Spinel is not only capable of connection but genuinely craves connection. He felt valued when Gibeon gave him special approval for his profile as a researcher and wanted to be special to him. His behavior around Friede also suggests he sees in him someone he could be friends with, but his fear of being hurt prevents him from making the effort to try. So he studies Friede from afar, trying to get to know him, and is fascinated by him because deep down, he sees a potential kindred spirit in Friede, someone he shares things in common with and could even be friends with, but he's too afraid to let Friede get close to him. Friede even recognizes this and calls him "Mr. Awfully-Shy".
His fear of being hurt in a way also foils Liko. Which is interesting that they're both Dark-type users and have similar but also opposite battling styles rooted in cunning, trickery, hiding, and deception, and overall being sly and devious. But painting Spinel in this light also paints a mirror to her because Liko is defined by her empathy and desire for connection.
Liko started out as someone who was very afraid of hurting others. She wanted connection but never got close to anyone because she was afraid of inconveniencing or upsetting others, and thus she would much rather hurt herself and put herself through suffering instead, so that she doesn't hurt others. Spinel is the polar opposite in that front. He also wants connection, but he's afraid of being hurt himself, afraid of other people hurting him, and thus he would much rather hurt others and make other people suffer instead, so that they don't hurt him. That foil kind of raises interesting implications for what kind of dynamic they could have whenever they end up directly confronting each other, especially since they're both Dark-type users and guile/tricky, but Liko's Meowscarada has a more bold and risk-taking type of guile, while Spinel's Umbreon has a more guarded and cautious type of guile.
In Spinel's ideal world, everyone is a two-dimensional figure that wears their heart on their sleeve. Nobody would develop or change. It would be a world of lies, living in the moment, with easy answers, and simple truths. You would never need to wonder about someone, doubt someone, or believe in anyone, hide anything or search for anything. It would be a static world without worries. Without progress. Without decisions. Without problems.
Which is why his public media appearances and manipulation of public opinion paint people in a black-and-white mentality. He paints the Explorers as heroic figures while he paints the RVT as villains, even telling some of his new recruits about Liko and Roy specifically and that they're evil. The people in both groups, however, aren't really that simple and easy to judge. Coral and Sidian are not "evil" but have personalities that aren't conventionally likable. Coral is a loudmouth who expresses her thoughts and feelings with no filter, even if the sweeter personality she shows to the masses post-timeskip is in a way, a real facet of who she is. But the public now only really sees a part of who Coral and Sidian are, not all of them, which traps them because they don't get to fully be themselves, having to actively hide their more..."unpleasant" traits to keep the masses liking them.
On the contrary, Liko and Roy face a public reputation as villains if their affiliation is ever outed. But they also have the ability to challenge perception. Roy is by nature the exact opposite of the RVT's reputation. He doesn't have a single bad bone in his entire body and is completely honest. Liko on the other hand is...well..."evil" in a way and her fighting style and team aligns with it, combined with her lowkey self-centered motivations, but she cares for others and has a good heart behind an "evil" mind, and is capable of showing it in a way. That instance with Nyabiko in Episode 93 where she calmed an angry flock of Dodrio and Spearow by putting on a magic show with Meowscarada, showing a more non-violent method to calm Pokemon, was a case of challenging a pre-conceived notion. Even though she's seen by others as an evil person, she can be more than that, and lets her actions do the speaking. Even a supposed "villain" like herself can be more than that and capable of doing good and being worthy of trust.
Spinel wants simplicity. He embraces a truth but fails to look farther or make any effort to challenge perception. He knows his peers in the Explorers see him as a despicable person not worthy of respect, and leans into it, but refuses to look further or aim for anything or accept that he can be more than that. And thus he lies to himself. Because that's simple. Easy.
But in reality, truths never are.