Responses

You're right.
Spoilers:
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He belongs to him. Shiki asked Ryuuzou one month of freedom. Ryuuzou beat him then agreed.Shiki told him he'd never ask him anything else. That's why Hauna and him can't be together.

*Haruna

Sorry. I was trying to confirm what you said. That's why I put spoiler. You can delete my comments .
When Haruna said 'Shiki's Dragon', the image is Ryuuzou's back.
Fyi, Ryuu from Ryuuzou's name is a kanji for dragon. So it's like with Shiki having that tattoo, it means he belongs to Ryuuzou. No, not in romantic way.