"No, I am not her boyfriend because I already have a boyfriend myself," Reo answered honestly. Reo didn't know why he misunderstood his relationship with Lisa while they didn't have anything special.
Reo was not shy to publicize his relationship with Arya. After all, it is a fact, and Arya is really his boyfriend, so he doesn't need to hide this fact all. Denying it meant that you also denied your boyfriend's existence.
Elena already taught him so much that he needs to respect your couple and never makes your couple upset. If Reo did that, then he had to wait for his older sister's punishment. The punishment is so simple, but it is a nightmare & hell for Reo.
Elena would refuse to acknowledge him as one of her younger brothers, and then she would only recognize Leo as her only brother. Reo couldn't accept this at all, and he promised his older sister that he would love & protect his lover in the future.
