Hermann's Links
After everything he'd gone through Hermann believes everyone deserves another chance regardless of what they've done as long as they regret their misdoings and want to better themselves. He knows about Reinhardt's past and everything that happened to him; now he just wants to see him succeed in his new life with his new familiy, which he is happy to be a part of.
Reinhardt's presence helps him to overcome his past so that he can give his best in his job as exorcist and in their relationship. He'd do a lot if not everything for Reinhardt, even going through hell.. again.. and again.
Mostly just romantic feelings and a deep respect for how Hermann accepted him even though his own past wasn't as peaceful as one might suspect. Reinhardt loves him deeply and found a way through his boyfriend to accept himself and what he has done to work through his former, twisted views.
Despite not being physically able to do much, Reinhardt would give his best to do anything for his partner. He never scares away from making sure their family is happy and safe, even if he puts his health at risk.
Even though Blake is older than him and can't possibly be his real son, Hermann sees him as one. He knows how hard the life as a demon can be so he wants to help his son, regardless of what he'd done in his life so far. Just because he had a hard life doesn't mean he shouldn't have a loving family and a loving father which are there for him if he needs them.
Sharing some interests as working on cars and listening to some similiar music genres just helps to strengthen their bond.
Blake feels a familiar bond towards Hermann as he was the one to provide him with a home and somewhat of a father figure after Blake returned to Scotland. Irrated at first and fearing he could destroy his newfound 'family' with the bad luck that seemed to follow him, Blake was surprised that Hermann accepted him without judging what he did or what would still be coming. The demon would never betray or hurt him.
When Hermann was still pretty new to the exorcist business he had a one-night-stand with a powerful exorcist after they finished a job. He never expected anything to come out of it and was never informed about his fatherhood but after a few decades he finally found out he had a son. Luckily Felix was by his side then so he could get to know him.
He quickly learned to love son and tries to make up for all the years he wasn't there for him.