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[Excerpt from Boku no Hero Adventure wiki]: 

Todoroki Shouto’s Witch King classpath is only accessible through the hidden third path of the Exiled Prince Story Quest. It is the only promotion that allows Todoroki to use both fire and ice magic (his King and Witch classes can only perform fire or ice, respectively). The Witch King path requires the Hero to fail the Kingdom coup, break out of prison to rescue Todoroki’s ice witch mother from a live volcano, and restart the revolution to fight a more powerful Hellflame King. If the Hero succeeds and if the Hero and Todoroki become romantically involved, Todoroki will give the Hero the Blessing of the Witch King, imbuing the One for All blade with both ice and fire damage and resistance. 

Todoroki then leave the party as per scripted, but will rejoin earlier than normal, in the middle of Act 4, during the Hero Killer boss fight. Completing the Witch King story line also unlocks Todoroki’s Plus Ultra ending as well as a touching Go Beyond epilogue if the Hero and Todoroki remain together through endgame.

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