I may be entirely wrong but I believe what Lena seems to have is a domination fetish. Now, BDSM encompasses things like bondage, discipline, dominance, submission and sadism etc. So It's not really clear the complete dynamics of what Axel and Lena's relationship was aside from the Dom and Sub dynamic between them. But from what I know, people in these kind of relationship have a great deal of trust in each other. Which is why a sub has no problem giving up control to their partner and trusts that said partner won't abuse their power. And the dom does everything in their power to make sure their partner is safe and are comfortable. Cause when all it's said and done, It is just a sex thing and the sub still wants to be treated like a person even if elements of said dynamic could bleed into everyday life. Which is why when Axel abused his position and betrayed Lena's trust, the relationship fell apart. But from what I've seen, I'm unsure if their relationship resembled a typical BDSM one. I think the one that closely resembles a typical BDSM relationship is the one between Lena and Louise.