@Lepper
Actually, there is nearly or no character development in typical Seinen manga. The main character in the beginning is always the same as the one in the end, he might take up a new skill or hobby, develop some muscles, but his character doesn't change.
In contrast, Shounen always has character development, they learn, alter or overcome their weaknesses and increase their strengths, their character traits change drastically compared to the beginning.
This is why most Seinen stories are boring,repetitive and sometimes even not conclusive, making their ending unsatisfying for most men.
Now, for this story, it has a Shounen part in the other dimension/planet where Chris comes from, and a Seinen part on earth, clashing both genres harshly.
But the reason why it is listed is Shounen is simply because it is released in a Shounen Magazine (Manga Box)