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Tearjerker: Tenkai Knights
Ever notice the generic looking Mooks that make up most of the Corrupted forces? The episode "Rising Fire" reveals that they used to be normal Corekai until they were captured and turned into Corrupted by Vilius himself, ouch.
Episode 16 "Oh No I Didnít" is full of tearjerkers, Ceylan has self worth issues so bad that he considers leaving the team again.
While on Quarton Ceylan and the Corekai are attacked by Slyger and Granox, who both transform into Titan Modes of their own and decimate the group. One particular Corekai who happens to be a big Valorn fan (he even shares the same green color scheme) gets killed mid-sentence.