When Lucas discovers that his long-lost brother Claus has died and is now the cybernetic, right-hand man of the main villain known only as The Masked Man. Then he is forced to engage in a solo battle (The Masked Man having fired a lightning bolt that wiped out the rest of the party but - thanks to Lucas' Franklin Badge - left Lucas unscathed) with The Masked Man, except he can't fight. He just can't bring himself to do it. All he can bring himself to do is curl up into a ball (ie. go into defend mode) and heal himself periodically, in a useless effort to try to stop his own brother from killing him. All the time, the ghost of their dead mother Hinawa is desperately and futilely begging Claus to come to his senses. Then their father Flint shows up to this scene of Claus kicking the shit out of Lucas' immobile form. Claus launches a PK Love (an incredibly powerful Psi attack) at Lucas, Flint intercepts and takes the blast.
Mother 3 said:
Then Claus launches
another one, this time at Flint,
his own father.
Then he resumes kicking the shit out of Lucas, this time with Flint joining in Hinawa's entreaties.
This goes on for a while. As the fight progresses, Claus starts to look around for the sound of his mother's voice - he can hear her. He slows down. Flashbacks to when they were babies occur. After a while, Lucas finally brings himself to fight. Not long after, Claus is clearly in great emotional distress, as he slowly comes to his senses. At this point, Lucas can still fight, but is so blinded by tears that he can't hit Claus at all.
At this point, I think it's best to leave it to the game's narration:
Mother 3 said:
The Masked Man removed his mask.
His face looked just like Lucas'
It was Claus.
Claus fired an intense bolt of lightning!
Lucas' Franklin Badge reflected the lightning back! [note: There is no doubt by this time that Claus is fully aware that this would happen]
Claus took mortal damage!
Claus staggered toward Lucas.
Claus embraced Lucas.
Lucas remembered Claus' smell.
Claus said:
I'm sorry it turned out like this.
I'm really happy you could be with me just before the end...
Thanks.
Dad.
I'm sorry I didn't listen to you.
I'm going to where Mom is now.
Lucas.
I hope we meet again someday.
Bye.
Thank you.
I'm sorry.
I'm sure we'll meet again.
Claus gave his final breath.
Keep in mind that with the exception of Hinawa's and Flint's begging (which is along the lines of "Claus! Can you hear me? Listen to me! Lucas is your brother!"), this is told entirely through the final battle, on the battle screen. And the music is just plain creepy, adding to the sense of wrongness about the scene.