Aren't Leitmotifs supposed to represent development and different conditions? In what way are the characters in the video connected other than the fact that they are in the same game?
Think if it more as a common song for a character, action or element that has several situational applications.
example:
Never give upDon't give upboth are scored as the same song, yet arranged to fit the situation, and both are a Leitmotif of
Undyne's desperation to avoid death. The former one being during the neutral playthrough and the second being the genocide playthrough.
Let's say you have Bob's theme. Every time Bob makes an appearance, the BGM switches to his theme to signify to the player that this is Bob. Maybe when Bob is mortally wounded, it plays a more melancholy arrangement of Bob's theme, or when he's possessed and going insane, it plays a more ghastly remix of Bob's theme. This would be Bob's leitmotif.
For more reading, check here
http://tvtropes.org/pmwiki/pmwiki.php/Main/Leitmotif