[Spoiler of chapter 9 of Final Fantasy Brave Exvius season 4]
In chapter 9 we see Vanharma's face.
As Rain and Roca state, he looks like Lasswell, but with blond hair and brown/red eyes.
So, I have two theories in my mind.
The first one, is that Vanharma too is a descendant of Yshe that landed in Levonia throught a portal, like Rain and the gang.
The second one, and to me the most plausible one, is that he is another Lasswell.
In season 2 we had another Rain, the one that saw all of his friends, family and loved ones perish by Vlad's hands; so, why not having a Lasswell from another alternative timeline where he lost everything? Probably not being able to save Physalis from merging with Omega and losing also Rain and Fina? It probably could be another reason why he is fond of Ihana: in her he sees the people that he loved and that he lost because of Sherna doings.
Also, he goes by the "kindness is weakness": kindness is one of the most visible qualities of Lasswell (even with people that don't deserve it), and seeing his world falling apart and his light fading, he could think that relying too much on that is a weakness.
Let's not forget that when he wounded Rain in the first chapters, he didn't meant to kill him, but to stop him for a while.
And, on the last note, in the Japanese version of the game they relased a new artwork of Lasswell where he seems to wear Yshe's armor but... if we take a look at the apron on his rear, it looks alot like the one that Vanharma too has. What if that version of Lasswell will be Vanharma's true form? Let's not forget that he learned how to transform in the Wyth village in season 2.