Complex sentence: Tsukasa isolates himself, but eventually he begins to get closer to other players, and builds strong relationships with some of them; the most important is the one born between him and Subaru, a kind and thoughtful female Heavy Axeman.
Split into simpler sentences: Tsukasa isolates himself, but eventually he begins to get closer to other players, and builds strong relationships with some of them. The most important is that which is born between him and Subaru, a kind and thoughtful female Heavy Axeman.