As a F2P what you can do is stack up those charms/refines/cave keys and once you got the max 500 at refines/charms and 1050 at cave key. You wait a rebate and spent them there. (You get refines back and charms too, so you don't have to be at exact 500)
For sure power you can get clothes. With time you can piece together them without spending anything on them. Also you can do the binding in your home and such.

https://forum-narutoen.oasgames.com/page/show-post-29504-1.html here is the post of what gives you power