Spur is the strongest demon beast Nightmare ever created. He is Terra's original half, and is nothing but pure evil, wishing for the ruin of all.
Spur is a dark grey Popopo with crimson-red eyes, black feet (with a red zig-zagging dash going down the middle of each), and the same red gem as Terra. He also has two black dashes seemingly painted onto each of his cheeks.
Spur, when completely unhindered by opposing will and unseparated from his good self, is possibly the most powerful being to exist within the realms of Zolara, easily surpassing even a large group of the DMR's elite, as well as Nightmare himself. The catch? He has never been given these conditions. Due to Terra's will, Spur is oftentimes significantly weaker than his full potential allows.
However, even when not in full power, Spur is a massive threat to anyone standing in his way. He is blindingly fast, strikes with massive force, and can shrug off an amazing number of hits. He can float in the air, and even launch his own projectiles at opponents. Not even Terra can come close to standing up to him on his own when the two are separated.
Spur wields (or more Wolverine-styles) the Nightmare Claws, a pair of pitch black claws that amplify damage, but offer no range whatsoever. Spur works extremely proficiently with these claws, easily being able to rush you at any point and end a battle faster than you can blink.
(A note: This sounds like a god-like character, right? That's because he is by nature. Terra's will or separation from Spur is the only thing keeping him from truly being completely unstoppable.)
It is all already explained, and more, here.
Warning: If you're hyper-sensitive (as I myself am), you may want to skip over this part as it goes into questionable detail for some not-so-pleasant things.
Spur only knows two emotions: rage, and cruelty. Spur is, more often than not, incredibly angry. He uses this anger to fuel his already impressive battling, and fight dauntlessly, confidently, and without mercy. However, when he finishes off an opponent is when he shows his main emotion...absolute cruelty. Spur doesn't just kill. He enjoys every second of it, often purposefully dragging their life longer and longer, savoring every struggle the victim makes in his grasp, or every turn of the claw he makes inside one's freshly cut-out gash... He doesn't just wish death upon his victims; he wishes to inflict the maximum amount of suffering possible, all while smiling straight into their soul as their vitality dips with each passing second. He has no care for life itself, and his gruesome killing methods prove so.
Relationships with Other Characters
As Spur's opposite half, he despises Terra. He is the only obstacle to Spur's ultimate success, and, no matter what, he is forced to be weakened by Terra's persistent and unwavering will. Even when in one body, and Spur gains control, Terra's will still persists, keeping him well shy of his full potential.
Terra's closest friend is, of course, Spur's greatest enemy, besides Terra himself, and his number one target, save for Terra himself. For obvious reasons, Spur wishes to harm the closest ones to Terra, only so he can watch them suffer and writhe in pain. With the two being brothers in every way excluding by blood, Kirby is Terra's strongest link; what keeps him grounded in his resolve. To Spur, killing Kirby off serves to 'cut the head,' so to speak, of Terra's will. Of course, he understands that the Star Warrior is not one to be taken lightly, even fighting alone.
Terra's father figure and mentor is another big target to take down for Spur, but he knows better than to simply rush and attack the war-hardened knight, knowing of his strategic advantages over his ruthless attacks, despite the physical advantages Spur has. He is secretly maddened by the fact that Meta Knight doesn't even flinch in fear at his presence, or feel intimidated in the process.