Again and again the question arises whether the Diabolik Lovers Vampires can only fall in love with Eve or whether they can also fall in love with someone else.
I found something interesting about this in Ruki's Dark Fate Route.
First, however, I would like to mention that these are just my thoughts. The English translations of various translation blogs are my sources. And of course it's a matter of interpretation.
Dark Fate Maniac 10 — Ruki's Monologue
I won't leave you, no matter what. I'm sure that deep down, I always wanted someone to tell me those words.
Many years passed by, until I met a woman who didn't just make false promises, but actually stayed with me no matter what I did or said.
Translation: @dialovers-translations
It sounds to me like Ruki tried to have a relationship before he met the MC. Those women made him promises but never stayed by his side. He wanted someone who would be honest and not leave him.
Based on this fact, I assume that Ruki, as a former human, is quite capable of loving someone else.
And again, this is just my personal opinion on this. Everyone is free to interpret this differently.