Hans is Einar's childhood friend who grows up to be good friends with Greta and Lili. He's tall and friendly with big hands—think Shrek as a human—but more importantly, Hans provides contrast to Einar. His gender identity remains constant, and so does his attraction to Greta. Ooh la la.
Throughout the time we know him in the book, Hans only has eyes for Greta. Despite the fact that Greta is married to his childhood friend, Hans doesn't feel one bit of remorse about being into her. Before you say, "That cad!" and slap him with a white glove, though, know that the reason Hans doesn't feel bad is because Einar isn't into Greta in that way at this point. The only woman Einar is into is Lili, so Hans feels like Greta is up for grabs.
Hans is one of the many levelheaded friends Lili has who helps her throughout life. As children, Hans helped Einar because he had a more stable life than Einar did and was slightly older: "Hans understood the grown-ups who ran the world: he knew they didn't appreciate their inconsistencies being called out" (3.12). It would be tragic if he then grew up to be one of these stodgy old grown-ups, but he doesn't—as an adult, Hans is unafraid to call out Greta and Einar/Lili on their inconsistencies, like telling Greta to leave Lili when her transition is complete because Lili is not the man Greta married.
Finally, there isn't any hate in this book, although we are told that Hans's father beat Hans when he caught Hans and Einar playing husband and wife as children. The incident sticks with Einar/Lili, but not enough to keep him from following his dream. Phew.