Don't worry about whether you're using the em dash correctly, 90% of people can't tell the usage differences between hyphens+en dashes+em dashes anyway. The other 10% are grammar nerds.
Her name should be Dys which comes from words like 異形成 (dysplasia) or 異形症 (dysmorphia/dysmorphism) which share the same root as 異形 in the comic title (異形ちゃんでいず).
I'll follow up on that and say that I am a grammar nerd who gives a bit shit about trying to use em and en dashes properly myself, but I don't even really notice or care how other people use them.
They're just lines, do whatever you want with them.