Uuuuuh so I'm going to try posting an imagine/headcanon thing for once! 😅 a lot of this is probably OOC so dont mind me lol
Robotnik is actually a big sweetheart!
In an interview with Jim Carrey, he said that Robotnik just wants to be special to somebody-- but its gone megalomania, he wants to be special to EVERYBODY. He absolutely ADORES any love and affection his significant other gives him. Even though he's pretty cold and a bit selfish a lot of the time, he wants to constantly be the center of attention with his S/O.
He needs a lot of reassurance, too, as underneath all of the confidence is a deep level of self-loathing. Jim Carrey even said so himself-- in a way, Robotnik views himself as just a speck of nothing in his own eyes.
He rambles a lot about his interests-- mainly his machines. He could talk for hours and hours about them, and he's always happy when people like to listen. He likes to snuggle in with his S/O as he talks, a lot of the time.
He LOVES music and dancing. He definitely has a playlist made specifically for him and his S/O. He likes to dance with them, either slow and affectionate or fast and passionate. He just loves it. When he's by himself, he likes to dance to help him think.
He DEFINITELY makes cheesy dad jokes to try to get his S/O to laugh. Or he does other things, like making goofy faces or telling them funny science facts. I'm sure he'd have A LOT of inside jokes with his S/O.