So far, all I have seen is a relationship based on lies and manipulation. He treats he like a doormat by making her do his work and has even feigned a back injury to do so. She hid a warp pad so she would be trapped on an island with her. Them making out was out of fear of never getting off the island and loneliness.
If I could compare their relationship to anyone else's, it would be the Joker and Harley Quinn's. Lars has Sadie believing that she is in love with him and that he loves her back, when the truth is that Lars is just using her. He is immature and cares only about how the world see's him, which was shown in "Horror Club" the best.
Sadie is a hormonal teen not completely in control of her emotions. I have a good feeling that this is her first relationship and still hasn't learned how to be more confident and less submissive to the will of the male role. As this is her first relationship she has no one to compare Lars to, thus can judge if this "relationship" is good or bad. In my opinion, Ronaldo is better for her.
So Lars and Sadie is a big NOPE for me.