it's a bl manga/manhwa (i cant remember) about a guy who got isekai'd twice; first is during his chuunibyou phase where he became a spoiled brat that cant use magic but keep being so arrogant, and the second one is when he's an adult. the mc has a jet black hair, and the ml is his knight? i think. they fought during the first isekai, then the mc got killed right after that.
I think it's about an astronaut that has this little flying thing that acts as his assistant. He has crush on his best friend (a scientist I think). He was planned to come back to earth after a year or so, but to his surprise, earth was destroyed a year ago. All the time he spent talking virtually to his best friend during his time on the space is actually a hologram(?) made from his little assistant.
https://www.mangago.me/read-manga/hate_no_kouya_de_vacances_wo/
https://www.mangago.me/read-manga/hate_no_kouya_de_vacances_wo/
Proceed with caution this was a sad read