Change Protagonist
Maybe you've got the wrong protagonist. What if the story was focused more on the half-sister, or the soulless girl? What if you told the story from one or both of their perspectives?
Get Him Invested
Alternately, if he really does need to be the protagonist, get him more invested. Why is he going to see his half-sister? Is it something he actively wants to do for his own motivations, or is he just doing what he's been told to do or thinks he's expected to do? When he meets the soulless girl, is he just reacting to her? Or does he actually want something in his interaction with her - eg to understand her, to figure out why she seems familiar, to get away from her, to date her, etc?
One idea I had, from just this summary, is that maybe he could start out aware that there's something he doesn't know about his life. He suspects family members are lying to him, he's experienced things he can't explain, he's asked questions and gotten no answers. So he decides to seek out his half-sister, who he hasn't seen in quite awhile, in the hopes that she'll be more forthcoming. This gives him a motivation and investment in the plot.
Perspective Character vs Protagonist
Usually, the perspective character is a protagonist, but not always. 248
Moby Dick is a good example - <PERSON>, the perspective character, is so forgettable I had to actively look up his name just now. The real protagonist is Captain <PERSON>. But the story works from <PERSON>'s perspective. It gives you the view of someone who is powerless while an insane authority figure derails a whaling expedition into a doomed quest for revenge that gets him and most of his crew killed. The story would have a very different tone if it was written from Captain <PERSON>'s perspective!
Character Growth
Another option could be to make his investment a character arc. Maybe he starts out just reacting, doing things for others without considering what he, himself, wants. And it gets him things he doesn't like, and finally, he hits his breaking point and decides to rebel. 190