Right click the glass: for 5 seconds, half the damage you take goes back to your attacker.
The Kagami does not take less — it takes everything, as before. It simply returns the favour. The difference with the Ishi is complete: the Ishi survives, the Kagami trades. Both can die to their own ability, for opposite reasons.
The reflection can kill, and the kill is yours — with its coins, its streak and its Elo. A kit that killed without anyone getting the kill would be a bug, not a subtlety.
The activation is deliberately loud: a silver flash and the sound of glass, seen and heard by everyone nearby. A good opponent stops hitting and waits it out. That is exactly the duel we want to create.