Cross and Killer are rivals. Killer sans is a flirt and a loyal but cocky right hand man to both of their bosses Nightmare.
Cross sans on the other hand is more of a realistic. He was a former royal guard back in his old au and prefers to keep a cold composure. Cross easily gets annoyed by Killer but as much as Cross hides it, he holds a somewhat special soft spot for him in his soul.

*Art by ProductionRoxy on Twitter, made for friend

A loner by nature, Knuckles is independent, headstrong, and serious, yet gullible and short-tempered to a fault. He is entirely devoted to his duty, spending most of his time on Angel Island protecting the Master Emerald, and rarely cares about anything that is of no importance. When his friends and others are in need of his help, however, Knuckles can always be relied on, being willing to leave his home very often
He Is Known for his powerful, wild nature. Hes a old friend of sonic the hedgehog

A hothead

Killer sans is a skeleton teenager rounding about the age of 13-15
years old. He’s quite the flirt even at this young age but wouldn’t go past a persons comfort zone. He’s very hard to read due to his poker face that always have the same grin on. He continuously has a black liquid dripping out of his eye sockets.