[Script] Help with HG/SS rom hacking

So i'm fairly new to rom hacking and I would like to make small little edits that'd be fun to make/get used to before actually making a rom hack.

I wanted to know how would it be possible have a Pokemon in the Overworld where you can interact with them and battle them. Sorta like the Legendary pokemon, but any pokemon I wanted to choose and any location. I already know of some of the tools like PPRE and the Lua XY Coordinate finder. However I believe it is very simple to say I have no goddamned idea what I am doing.

Would there be any Specific threads that could help me with this? Videos? etc? Thanks in advance!