Just a reminder that providing specifics on, sharing links to, or naming websites where ROMs can be accessed is against the rules. If your post has any of this information it will be removed.
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I don't think this is an easy one. I think if you have a chance at making this you would have to use ASM. Or a cheap way (if the sprite is small enough and you want a permanent pokemon.) Try an OW editor with the pokemon right behind the char sprite
This thread should be able to give you a good grasp on on how to work with a Follow Me feature. I think it'd be worthwhile to read the entire thing because there is in-depth discussion on the pitfalls associated with Follow Me.