Though he isn't seen in the opening video, Gateau took part in the attack that led to Cube taking your daughter to modern-day Japan. In fact, he was the one who murdered her birth parents.
In April of your daughter's 12th year, she'll hear a strange bell jingle outside her house. When she goes to investigate, no one's there. The next Sunday, send your daughter out to town. When it's time for her to come home, she'll get lost and Gateau will find her. He shows her a pair of bells that he has with him, which give her a powerful headache. This is because the bells were a memento of your daughter's mother, and seeing them triggers her past memories that had been wiped by Cube. Gateau tells your daughter that he killed her true parents, and he's about to kill her too when you arrive on the scene and defeat him (tearing one of his ears in the process). Gateau disappears, but your daughter has the option of fighting him once a year after certain story events have taken place.
As she encounters him, he always speaks about how he wants to dismember her, especially mentioning taking out an eye (in exchange for the eye he lost, one might assume), cutting off an ear-- he even goes so far as to say that he wants to pluck off her fingers and toes, one by one. This doesn't change, even when he eventually confesses to having fallen in love with your daughter. This naturally frightens your daughter, and she is able to fight him with renewed vigor. He continues to lose his sanity, even telling your daughter that her attacks were signs of her love for him.
When your daughter fights him in Heaven, his body has deteriorated and he's barely the man he once was. Your daughter still demands that he apologize to her dead parents. He does, though it sounds insincere and he asks for her affection right afterward. She tells him that what he did was unforgivable, and fights him for the last time. His body crumbles apart, and his last words are, "I love you."
"One eye, one ear, and all the pain you gave me... This is the whole of my present. This is my offering to you."
"What is this feeling... is it love? Yes... yes, it is! It's love! I just hadn't noticed until now! ... I'm in love with you."
"Love is a wonderful feeling..."
"Soon I will make you mine! Now... why don't we become one?"
"I am so happy...... For you, this is the testament of your new love......"