To be sure of somebody - to trust somebody; to know that someone will always be there for you.
You are my friend I can rely on.
If the sentence is from 'Winnie the Pooh' by A. A. Milne,
Piglet wanted to know whether Pooh would leave her or something else.
The above opinion is only my personal supposition. I wanted to be sure if you took my advice.
- For Teachers