[Seerah]

function lf:OnEvent(event)
  if (event == "ADDON_LOADED") then
    CheckNewChar()
  end
end
lf:SetScript("OnEvent", lf.OnEvent)

This is perfectly fine. The function lf:OnEvent makes OnEvent a method of the frame lf. When you assign lf.OnEvent to the event handler, self and event get passed through automatically. Using lf:OnEvent means that lf *is* self, so only event is needed in the parentheses.

Now.... He does still need to check if it was his addon that loaded.