I'm not entirely sure who Erika and Klaus Mann are specifically in this story without more research. But they are likely the main characters. They could be siblings or just two individuals bound by a common experience in the context of this 'escape to life' story.
Klaus is one of the main and complex characters in The Vampire Diaries novel. He is an original vampire, which makes him extremely powerful. His character is often depicted as being very cunning and manipulative. He has his own motives and agendas throughout the story, which usually involve power and survival in the dangerous vampire world.
One possibility for a great love interest for Klaus is a human. A human would bring out the more humane side of Klaus. The contrast between his immortal, powerful self and the mortal, fragile human would create a lot of interesting dynamics. The human could be someone who is initially afraid of him but then sees the good in him. Also, an original vampire from another family could be an interesting love interest. This would involve a lot of power plays and political intrigue within the vampire world. They would understand each other's nature and the responsibilities that come with being an original, but at the same time, there could be rivalries between their families that add to the story.
It's a complex situation. Different storylines and villains have targeted the Avengers, but no one specific character can be said to have killed them all outright.
Kang's demise in comics is a complex matter. It could be a result of a team-up of heroes, or it might be due to an unexpected turn of events involving multiple characters and their actions.