However, we know now:
Support: "Triggers when another ally monster attacks."
Ranged: "This monster won't take counter-damage."
As such, Ralsei is a very good card, able to make any of your other monsters unable to take DMG when attacking. And due to Ranged staying on the monsters, you can combine it with cards like Doggo or Nacarat Jester in order to stop its own Dust, which would remove the Ranged.
One of his most powerful combos, however, is with Papyrus. Papyrus with Ranged can be extremely difficult for your opponent to deal with without losing a lot of tempo, since he can just rush down your opponent's monsters without losing HP.
Best used when you're about to attack with monsters already on the board, as Ralsei is pretty frail itself, being weak to Muffet's Pet.