This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Kamen Rider Kiva
Episode 11 shows how Yuri's mother died at the hands of the Rook fangire.
A few episodes later it is revealed that the same Fangire slaughtered Jiro's wolf tribe.
Dai-chan/ the amnesiac Rook Fangire killing the girl who worked at the Soba shop who he had become close to after he regains his memories.
The deaths of both the Fangire husband and sickly human wife from episodes 23 and 24. The wife knew that he was a Fangire all along and accepted him for who he was. She dies after they visit the memory tree they planted in 1986 and crosses the Despair Event Horizon, snapping and hurting Mio, causing Wataru to tap into Emperor Form for the first time and killing him in what is presumed to be blind rage. He shatters as he holds the hand of his wife.
Kengo no longer being able to play guitar due to an injury to his arm. Wataru not telling him this even though he knew about it causes him to end their friendship. He later returns as a no-nonsense member of the Wonderful Blue Sky Organization and takes IXA away from Nago.
He later apologizes to Wataru for his abrupt change in attitude and asks him to be his friend again. He reveals that he knew that he never had any talent for rock music and was relieved that he could no longer play. He used his attitude to cover for his feelings of weakness.
The situation with Kengo, as well as the fact that Mio is Taiga's fiance causes the Bloody Rose violin to sustain a large crack due to the fact that it is tied to Wataru's mental state.
Mio's indecisiveness causes tension between everybody, leading Taiga to cancel their wedding and overhear her confess her love to Wataru, leaving him crestfallen and Kengo to admit that he likes her before being rejected and punching Wataru in the face.
Maya/Queen's having to abandon Wataru as a child because of her love for the human Otoya.
Wataru returning to his reclusive lifestyle after it is revealed that his mother is Maya, the former Queen of Fangire, making him half-Fangire and a perceived threat to humanity.
Shima ordering all members of the Wonderful Blue Sky Organization to kill Wataru/Kiva.
Taiga turning Shima into a Fangire, as well as later killing him in front of Wataru, which caused him to go into a rage and led to the fight that killed Mio.
King removing Queen's Fangire powers and exiling her because she fell in love with Otoya.
Wataru attempts to use Wake Up Fever to attack Taiga, who had earlier been wounded by Mio. Because Taiga let Mio escape, she steps in to protect him and takes the attack. She shatters in Wataru's arms. It is later revealed that Bishop is the one who killed Mio after she took the attack and fell off the cliff.
Wataru later travels back in time in an attempt to break up Otoya and Maya and prevent himself from being born just so that Mio would be alive again.
Otoya visiting Yuri, Jirou, Riki and Bassha and Maya before dying in Maya's arms.
Taiga killing Maya after he loses everything.
Although it is revealed later that this didn't happen.