this is very late but i'll give some extra info (as someone who uses Sayu a lot) rainslasher is often used on Sayu because of the Elemental Mastery substat, so along side the extra damage against enemies with Hydro or Electro from rainslasher's ability (what Rathurue stated), Sayu also heals more because of that Elemental Mastery due to her 1st passive healing the party for 300 + 1.2*Sayu's Elemental Mastery stat
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