From the elements you chose lightning probably works the best with shado . If it wasn’t limited to those four however I think light would be the best fit. I could see Rishi use light to create shadows to hide in or fight using a sword made of light to confuse his opponents. Also, it’d be quite an interesting irony seeing someone who is as cruel and petty as rishi have an element so aligned with good. End of the day however, it depends on how you want to take the character and story forward