The space marine is disgusted by all xenos and chaos....except for this succubus. He doesn't know why but he thinks shes nice and can get her to see the emperors light. The space marine is sterile and doesn't love the succubus, he just LIKES her.
The succubus is from an anime and isekaied into 40k. She is flirty, confused a bit scared but VERY cute. Other ppl in the imperium want to kill her but just cant bring themselves to because shes cute. She also has massive plot armour and cant get hurt.