This seems to be either something for people to contemplate on, or a flaw in the design.
Here are my theories:
Hypothesis: The animation represents the soul being torn from the body. It is then reconstructed.
- Undyne says “this is your last chance of redemption. Give me your soul, or I will tear it from your body” which suggests that removing a soul by force will look dramatic.
- Asgore’s soul breaks in the same way as your’s, when you absorb it in a neutral, or genoside run.
- When you kill a monster, their souls shatter instantly, but you would still expect to see the 4 particles fly from them. This could mean that the 4 particles show that a soul is being absorbed/obtained.
- Torial’s soul breaks in the same way, but it is unlikely that you absorbed it, as the Asgore fight plays out exactly the same when you kill her. (Frisk/Chara may may have felt obliged to fight, as Asgore wanted to take his/her soul.)
Hypothesis: Frisk/Chara had a monster soul the whole time (Asrial’s soul)(still a human body).
- We know that Frisk/Chara is a reanimated corpse from the pacifist and genoside routes. The light blue soul probably belonged to him/her. In the neutral run, we hear about Asrial’s tragedy: “Clutching the human, he smiled and walked away” which suggests that he was holding Frisk/Chara when he died. Asrial’s soul could have been absorbed by Frisk/Chara at this point, while Asgore obtained Frisk/Chara’s soul.
- It is confirmed that Flowey has no soul throughout the game.
- There seems to be a link between Flowey and Frisk/Chara. Furthermore, DETERMINATION seems to be bound to souls, so since Flowey received DETERMINATION from Alphys, it could have been passing from Flowey to Frisk/Chara.
- Asrial is a boss monster. If Frisk/Chara had Asrial’s soul, this would explain why the soul shatters the same way Asgore and Tori’s souls do. No other souls break this way.
- Frisk/Chara’s soul is a different colour to all the other souls. (This could be simply because Asrial/Frisk/Chara’s soul represents the traits of the typical RPG gamer, while the human soul that the other monster souls come together to make, represents Toby Fox.)
- This would leave it unclear as to how you cross the barrier in the neutral run. (However, we’ve seen how the player’s actual soul is part of the game. The creator did really well at making the neutral run seem so real the first time you play. This could represent your actual soul becoming temporarily bound to Frisk/Chara, allowing him/her to cross the barrier in the scene before the omega Flowey fight. This would probably explain why you can’t walk back across the barrier after the fight.)
If anyone could find a way to make the points easier to read/understand, without changing what they mean, that would be great.