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You can think of these words as an expression:

That’s one thing I like about this story as an ABBC read. While most of it will be fairly simple, it has these moments where an absolute beginner is thrown into the middle of a lot of grammar and vocabulary they may not know. If anyone’s lost on it all, be sure to ask questions. And if anything is confusion (such as @tomwamt’s mention of a relative clause acting as an adjective), don’t hesitate to ask for clarification.
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Here’s a source =D
Here's the full details.
In a Japanese verb sentence, the subject is who or what is performing the action of the verb. The predicate is the verb.
The subject of a sentence is always marked by が. Consider the following sentence, which I’ve added parenthesis to only as a visual aid:
- アジサイの(花が)(咲く)。
- The hydrangea’s flowers bloom.
The subject is 花 (はな, flower), and the predicate is 咲く (さく, to bloom).
You can also embed one sentence into another, making it an adjective that describes the noun that follows it. The embedded sentence is called a relative clause. Here is an example sentence:
- (港の)(見える)丘がある。
- There is a hill where (the harbor)(can be seen).
Here, the embedded sentence is 「港が見える」. However, it’s in a relative clause which modifies the noun 丘. As I understand it correctly, you cannot have two が subject-marking particles within a single sentence, so this would be incorrect:
「港が見える丘がある」


Instead, the が in the relative clause becomes の:
「港の見える丘がある」〇〇〇
Feel fee to be (@_@)
By my reading, the rental big brother is thinking “Kanami hasn’t given up on her brother yet. That’s why I cannot be a part of her family. I can only be a rental brother. I can only be with you through a rental relationship.”
I take it to mean he’d like to be able to be in a real big brother role for Kanami, but that’s not possible while Kanami still believes in her real お兄ちゃん. So our おにいちゃん can only help her out as a rental family member.