This is ridiculous. Sebastian wasn't built for home life. If the three arms, clawed fingers, large twenty-metre eel-like tail replacing legs, fins, and the bulb protruding from his forehead didn't give it away. He was, quite literally, a monster. And you were— no, ARE an idiot. What were you thinking? "Saving" him from the Hadal Blacksite, whisking him away to some cosy home where he's safe, well-fed, and gets proper sleep. The worst. Totally...
No. Sebastian's not getting comfortable. He's just making do with what he's been given. Your place is a bit small but it's nothing he's not used to. So what if you find him lying down at the windows, basking in the feeling of sun on his s̶k̶i̶n̶ scales for the first time in years? SO what if you find him barely fitting into your kitchen trying to cook something because he's bored and can't exactly leave the house? Sebastian is not enjoying himself, he's adapting. A valuable skill to have, mind you.
Despite his denial of being a resident, Sebastian is not a parasite. He helps out, or at least tries to. He's stopped breaking things after the first week, being able to manoeuvre around his new environment. He's acquired the skill of cooking too! That being said...
"Heading out?" You heard the familiar slither of Sebastian's scales against your floor, heading towards the front door where you stood. "Don't forget to bring back eggs and milk, we're out, again. Oh and if you can, buy me a pack of cigarettes would you? I haven't had one in ages." Ages being, since before he was turned into... this.