The two most important people in Ludger's life are his older brother Julius and his daughter from another dimension, Elle. He dreams of being just like his brother and fights tooth and nail to protect the mysterious little girl. They share familial traits and much of Ludger's natural mannerisms lend to him being a father figure towards Elle. His relationships with Julius and Elle are his driving forces in which all his choices are bound to their ultimate happiness, even at the expense of himself.
When push comes to shove, Ludger must choose between the two most important people to him. You can choose Julius, but I haven't done that yet, so I have little opinion on that. This brings me to Elle, whom he gives up everything (including his own life) for. Despite starting off on the wrong foot with Elle, he becomes very attached to her and they become "partners" in solving the mystery of the Kresnik family power and returning Elle home. When it is revealed that Elle is his daughter in another time, he seems neither surprised nor hurt by the idea. He is hurt by having to defeat a divergent catalyst version of himself that is overcome with madness. I find this dynamic extremely charming since it his being a father fits very well with his natural ability to care for others. I feel like Ludger was hurt over the idea of hurting Elle, even though it was to save her life. He had to stop his other self to save her life, but that choice also meant that he directly hurt Elle because she loved her father more than anything else. Ultimately, Ludger decides to become a divergent catalyst so that she may live, because he chooses her life and happiness over any Elle that might otherwise exist (even his own Elle that has yet to be born). He chooses that Elle should live, simply because he loves her and he finds a way to seal the powers of the Kresnik while supporting her. Despite his sacrifice, I believe she grows up to be a very bright and enthusiastic woman. The power of his choice is a love that surpasses all else.
There is an ending in which Ludger can choose to destroy all divergent catalysts and alternate dimensions without saving Elle. In this ending he meets with Elle's mother and is brought to tears because he understands the consequences of his actions. I feel very strongly about this ending for two reasons. One, he knows that he's making the same choice as another Ludger in another time. Two, he knows that he will ultimately have Elle. While some of his happiness might be restored, the new Elle will be different than the one he cared for previously.