It takes several years for Bastien to stop reflexively antagonizing the people around him and even longer before he begins to truly integrate with the rest of the team. This makes sense when you remember that Bastien is an empath and can feel the emotions of those around him. It's hard to play nice when you can physically feel how much everyone around you hates you and wishes they were allowed to kill you.
Initially, it seems weird that of all the Immortal Guardians, Seth would be the one to fall in love with a regular human who can't be transformed, particularly given how badly it turned out the last time he fell in love with a mortal. Then it's revealed that Seth is secretly the progenitor of all the Gifted Ones. If he were to have a romantic relationship with a Gifted One, he would be knowingly having intimate relations with one of his descendants.
