Don't Bother Him, He Won't Bother You
Probably the second most reoccurring Eon from Kirby's personal clan of nutty monsters, Umbreon looks a lot more menacing than he actually is. Despite the hairstyle which makes him appear to be either an emo brooder or an edgelord, Umbreon is mostly just an apathetic uncaring loner who doesn't want to be bothered by anybody for no other reason than legitimately enjoying being alone. Because he is usually indifferent, his tolerance level is actually quite high, and it would have to be when you're dating somebody as aggravating as Espeon. Umbreon lets a lot of minor annoyances slide while he barely shifts expression or demeanor. Much like Charlotte, he can sit through a lot with a blank expression on his face, or off in a dark room blankly staring at a wall. Unlike Charlotte, Umbreon is also snarky and opinionated, and isn't exactly curious of others. Even if he lets the stupidity surrounding him roll off his back, he's not one to go without sarcastically commenting on the actions of those around him. On the converse, however, his indifference keeps him from being excited about good things or good news as well (unless the news is that everyone is going to leave him alone).
Just because Umbreon is usually too lazy to put energy into getting upset doesn't mean that he should be underestimated. In fact, Umbreon should not be trifled with. Umbreon is the strongest of the Eons and the strongest Pokemon Kirby had altogether due in part to his ability to tap into his own evil power and supernaturally enhance some of his abilities. For example, Umbreon is able to create voids and rifts in space time, and when that darkness within powers him up, his Shadow Ball becomes more than a ball of dark energy and instead becomes similar to a gravity bomb, exploding in a void that pulls everything towards it and rips them to shreds from all the dimensional twisting and tearing. Umbreon, however, does not have control of the part of him that makes him an Evil type Pokemon, and that power only comes up during extremely specific circumstances, or if pushed in anger enough to the point where he loses himself and lets all negativity consume him.
One such example of cosmic circumstances altering his demeanor into something far more devilish caused him to thirst for blood and kill as many as he could get away with before looking for a pure sacrifice, which Flareon (who previous to this, joined him in his rebellious shenanigans) tried to stop him from doing. In a way, this behavior is similar to that of a werewolf, but far more rare, and was directly taken from the behavior of the Dark/Evil type Pokemon in Poké Legends, which is why Umbreon is a direct alternative to his Evil-affiliated counterpart (at least up until the Poké Island series came to a complete end).
Umbreon doesn't respect a lot of people, and at best only somewhat respects those who deserve it, like Kirby. However, this does make him unreliable unless the reason for his being summoned is a direct threat against his own sense of peace (or "I'll do it when/if it affects me"). Umbreon seems to really disrespect Death Reaper JOE, however, and gets annoyed at him faster than anyone else, perhaps in part because, unlike Espeon, Death Reaper JOE should know better and has self-awareness, and yet completely turns into an annoying fanboy over Umbreon, thinking he's ultra cool and constantly schmoozing up to him. Umbreon seems to break character at times over pretty girls, though, as he has shown himself to go big eyed and wide smiled over Flareon (referring to her as a babe or hottie), and even on occasion tried to woo Espeon when he's distracted and momentarily forgets who she is or what his default personality was supposed to be.