1. The support I assume is the stationary phase is C18 which is octadecylsilyl a non-polar stationary phase.
2. I'm not very sure what you mean by chemical nature, I suppose it is a non-polar chemical nature?
3. Again I'm not too sure what you are referring to here.
4. The column is non-polar C18 while the solute is also non-polar in general?
1. I think that you need to do some background reading, because a support is only a support while a stationary phase is only a stationary phase; they are not the same.
3. What chemical changes can you make to the reverse-phase, such that it is still non-polar ??
Again, some background reading---look at different reverse-phase chemistries offered by, e.g. Agilent, Thermo, Waters and other manufacturers of HPLC columns. How do they make their columns ? What is the support ?
4. Different stationary phases, different carbon loading on the support, different surface coverage ?
Keep going, this is not rocket science---you just need a bit more background knowledge of RP-HPLC, all readily available via the internet.