Although it's mostly associated with ash-tree in Bulgaria, the name may sometimes be given to mean "clear, serene", as ясен means that in Bulgarian as well. [noted -ed]
It would be really cool if someone where to name a boy/girl twin set Yasen and Kalina. In Old Norse mythology it was believed that women came from the rowan tree and men came from the ash tree. Kalina means Rowan tree and Yasen means Ash tree. That's kind of an interesting match.
There is a poem by Peyo Yavorov, Bulgarian poet, about Kalina (rowan) and Yavor (sycamore), called Неразделни (Inseparable would be the literal translation, although I'm not sure what's the title of its English translation).